On the last day of my old life I closed and locked the office door behind me, handbag knocking hard against my hip while I turned the key, that one extra click to ensure it was double bolted. There was nothing out of the ordinary, nothing to indicate a breach in the predictable pattern of life except that I was in a rush, and now I don't know what for, or why. To meet someone important, or to catch a train?

I've always been good at closing doors, those invisible portals between one stage of life and another. I suppose this particular talent of mine, this ability to effect closure, at least within my own mind, is what helped me to shut the door behind me and move back into my own life at the end of every working day. Not to do so would mean drowning in the sorrows of others, and I have a horror of death. The transition isn't always easy. To sit for hours listening to patients, to witness raw grief and withstand the hot tip of molten anger are the everyday fodder of my profession. Now I know I was hiding behind their misery — behind that door again, where I could pretend they were all so much worse off than me, and where I truly believed that I could be of some use.

Hubris is simply a shield against the worst fears about ourselves, and I speak with some authority.

But back to that door, the one at the end of that particular working day. My office is on the top floor of a Victorian house which I share with two other therapists and a dentist on the ground floor. Careful down the steps, my hand running smooth along the banister, I remember registering that Daniel and Martha were still in their offices, "Do Not Disturb" signs hanging on their doors.

As I passed Keith's office door, the scent of clove oil and tooth decay wasn't so strong as on other days, but there was still the scream of the drill, a little muted through the thickness of the walls. I remember wincing at the sound. As ever, I was pleased it wasn't me in the chair.

There was nothing to mark out the day, nothing to warn of the disturbance to come. The familiar soft pad of the rubber on the stairwell, and the usual flash of irritation that the patch at the bottom had not yet been repaired. I didn't know that these were the last moments of the life I had worked so hard to build, closing door after door behind me to ensure my safety. Now I wonder which door it was that I left hanging. A Greek patient once told me she never entered a room without considering whether there was a window through which she might escape. So perhaps that's it; I never considered how pain will leak through all those unseen crevices, or secrets seep out from the smallest window I neglected to close.

Blind to what lay ahead I simply walked out into the evening, a soft spring night that might have been beautiful. I remember the cool air, and the soft thud of the front door as my life closed behind me. I remember looking over my shoulder just once, an afterthought perhaps, or the pang of a lover left behind in a moment of longing. In the nether reaches of my heart I always knew what my mind never dared to admit.

I had a life, a good life even. I walked out of one door and passed through another, time in reverse, like the joke clock I had hanging over the door in my kitchen, forever confusing. I noticed others staring at it sometimes, their eyes squinting in puzzlement. The clock threatened their order of things, demanding a new view of the world. Funny thing is, I was used to it and could tell time in a heartbeat.

"What time is it, exactly?" Frank asked the first time he saw it.

"Ten to eight," giving it a cursory glance. I was pouring coffee after an early dinner. We were intending to go to the movies. "Just reverse the numbers. It's easy."

"For you, maybe. Why would you have a clock like that? What's the point?" Frank shook his head. Not for the first time, I noticed how much hair he had, unusual in a man of fifty. A little bit of grey in the corners, but mostly a dull blonde where the signs of age could get lost for a while before showing their true colours.

"What do you mean, what's the point? It's a joke."

"Why not play it straight? It's unsettling, if you ask me."

"Nothing is straightforward." Handing him a cup of coffee.

"Oh. Well, then ..."

Who had given me the clock? It's not the sort of thing you buy yourself. I lean more towards clothes and music, a good hotel with room service. It was a gift, the parting shot of one of Frank's predecessors, a man like the others who finally grew tired of working through my past in the present.

"How is it?" this man had complained, handing me the clock wrapped up in pretty white tissue paper. "How is it, that someone who analyses every little thing can be so unreasonable? I think you need to figure a few things out," he suggested, not entirely unkindly. What had I done to cause him to say such a thing? Blast him for some minor infraction that replicated a larger injustice from my past, no doubt. He couldn't have known, but he'd given me the clock, so he'd registered something. I closed the gate behind that one, forgetting his name in no time, but I'd kept the clock, hanging it over one of my precious doors.

To Frank, so early in the game, the clock did not yet make sense — but it would soon enough. What I hadn't known then was that Frank was a stayer. He might not have grasped yet the reverse mechanisms of my life, his thinking was far too linear for that, but not understanding wouldn't be reason enough for him to leave.

I like buses, where everything is above ground, so that's where I headed when I left the office that day: to the stop at the end of the road. I probably figured it would take me where I wanted to go, and with less effort as the Underground is further away. I take the bus most days. For a woman who spends her working day tunnelling through the unconscious maze of others' experience, I am adept at avoiding my own. I take the Underground only when pressed, when there is little alternative but to head down directly. I like to sit up top on the bus, sometimes at the front like a child, facing the world head on. Down below, I usually end up on one of the "priority" seats, not because I'm old or disabled, but because it's the only one free. Already I feel cramped, pushed in on the sides of my life and forced to focus at close quarters. Up top I can breathe. Down below the air is thicker, people too weary to tackle the stairs.

The Underground always demands work. It's a fifteen minute walk down the road and I take it only when I'm forced to, when it's the only possible route to get where I'm going, for instance the furthest reaches of north London, or Dagenham, which I consider the end of the earth. Anywhere else I can skim along on the surface. Shoved onto the Tube I feel like I'm caught in the web of other people's misery. I am a psychotherapist by profession, well known in my field, but at the end of a fifty-minute appointment I am let loose. I can write my notes and re-calibrate my internal settings. On the Underground, you don't know who you might end up with, trapped in the middle of a tunnel with the lights out and no means of knowing how long you'll be there. How do you reach the air in an underground tunnel, except by passing through more tunnels? The maze is endless and there is no map, just a relentless series of dark turns without a single hint of what might be ahead.

I finally entered therapy, initially for a couple of weeks, because I supposed it was only a bit of fine-tuning I needed. This was after I was given the clock, which was really the last straw. In my innocence I believed it was a few unfortunate habits I needed to break, like giving up smoking, or putting that foul tasting stuff on your fingernails to stop yourself biting them, taking away the sweet comfort of gnawing at your own flesh before the pain sets in and you're reminded how disgusting it looks.

I ditched my first therapist when she dared to suggest that I was resistant. I wanted someone to show me how to negotiate turns, and she was all for slowing down and working out the route. Couldn't she see I was terrified of the dark? Like I said, I dumped her. I wouldn't have been able to stay up top with this one. Her cultivated kindness irritated me and her empathy was unsettling. She wasn't pragmatic enough, like an inefficient sales clerk who clucks around behind the desk while you're waiting for her to serve you; such a simple transaction turned into a drama. When I told her I was leaving, she lifted her slim shoulders and shifted a bit in her chair, pushing herself forward a fraction before speaking, as if she thought I might miss the point. "Have you considered why you want to leave?"

I'd already prepared my answer, "Because I don't think this is helpful." Eyes popping open, she looked gratifyingly startled. I just wanted to know how to manage my anger sometimes, how to get through the day. "Maybe this is for some people, but I haven't got the time."

Lifting her hands she gripped the wooden arms of her chair either side, as if to steady herself. She didn't argue. She smiled, a tight little line that settled the deal. I was out the door within minutes, into the light.

Now, I recognise that when someone is determined to leave therapy, there's no point in arguing. You become better at knowing the difference between a threat and an intention. She was wise enough to know that argument was useless, she could only encourage me to reflect. Later I would be grateful, but in that moment I was just happy to be out of her door.

I wasn't thinking of doors, the ones I'd closed behind me, while I was waiting for the bus that evening after leaving the office. I didn't have time, as a number 87 came along pretty quickly heading towards town. It must have done because I don't remember tapping my little internal foot until I actually got on the bus. Who was it I was meeting, and why is it that I focus my impatience so often on this hole in my memory? It wasn't the main event. That night was simply the warm-up act no one recalls afterwards, unless it was truly terrible. Except I do remember the ride, like the last view of someone you love.

The bus was crowded, and like always I headed up top. There was only one seat available that I could see, somewhere in the middle on the right. "Excuse me," I said, clinging to the pole for balance as the bus pulled out. The seat was occupied by a bag of groceries, their owner staring out the window.

"Oh!" turning towards me in mock surprise. "Sorry." She heaved them over and onto her lap like a ten-ton sack of potatoes. A martyr, I thought, a woman to be avoided. I plopped myself down and focused on her insignificance, my usual method for annihilating the enemy.

Two stops later she got off, huffing and puffing with her big load and I wondered why on earth she hadn't remained below stairs. When she'd gone, I slid further down next to the window. Someone had left a Coke can behind and it rattled back and forth underneath the seats. Without a litter bin, where do you put such a thing? I picked it up and shoved it between the seats. Two people were talking loudly on their mobiles, having competitive one-sided conversations no one, least of all me, wanted to hear. I glared a few times and one woman actually stared back, shunting her chin forward in an "I dare you!" sort of motion while her voice grew increasingly louder. The other was so self-absorbed she didn't bother to look around, head bent and phone tucked expertly between her shoulder and cheek, filing her nails as she spoke. They weren't worth my time worrying about. I turned to stare out the window, to see the familiar trace of shambolic pedestrians heading home after a day's work, the shopfronts and sooty brick buildings.

The bus rattled along the Wandsworth Road, not a pretty place at any time of the year. It always looks dusty, as if no one has bothered to clean the corners since the last bunch lived here, before the houses began to fade into disrepair and the junkies moved in. But there is always hope. From out of the window I registered the Congregational church, with its slogan outside promising salvation. I also noticed one more section along the route having a face-lift, a row of almshouses with scaffolding up and a sign to say luxury flats would soon be available. I wondered who would be the first to move in, imagining an aspiring City boy with his leather couch and a television the size of a wall.

The route was reassuringly predictable, except in pockets where the looting was a few years ago. I hated to see the boarded up shops, those small businesses that were never able to recover. What did they do afterwards, those people who had created a life in the form of a grocery store, or a hairdresser's? Some of them were refugees, survivors of other war zones. In the hard-won effort of establishing a shop, they had believed they were finally safe. I looked out the window to the other side. The thing about loss is that you can't replace it; so much trauma symbolised in those abandoned shopfronts.

Council housing runs like a ribbon on the left hand side of Wandsworth Road, while just a short distance behind the derelict buildings and the messy business of transition on the right is the stately elegance of Clapham Old Town. There areno Portuguese coffee shops here, with their twenty-four-hour football and men smoking and shouting through the open door at the television. Bakeries have turned into patisseries and the butchers are all organic.

I live between the two worlds, equidistant from the gritty, noisy road and the quiet confidence of Old Town. The bus stopped and started, with me up top heading towards — where? I passed my own stop, I remember that, and the double-decker continued on its route towards town, turning left at Vauxhall Bridge and up and along past the Tate Gallery and the House of Commons. I had a book in my handbag which I never looked at again. For weeks and months afterwards I was distracted. Instead, I watched history glide by and looked through the tourists to see if there was anyone I knew. There never was, but I always looked and I was always disappointed, as if somewhere in the crowd I should experience a flash of recognition, a resurrection of sorts where someone from my past might be pleased to see me. I sat up straighter, a child plastered against the glass to get a better view.

I must have landed somewhere near Trafalgar Square because the bus turns around at Aldwych.

The evening ends for me there. I don't remember the space between, or the journey home. It's the insignificance of the occasion that is so striking now. My life was about to tip over into my past and I can't now remember what I did the night before, like a drink too many and the hangover of regret. And no one has filled in the void, an evening too pedestrian an occasion to ever refer to again. Dinner with a friend perhaps, or a professional meeting? I have many more colleagues than I do friends, and they are easily left behind.

Somewhere between Trafalgar Square and home again I lost the last evening of my old life, and no one noticed, least of all me. Months would pass before I dared to admit it. For once, with the door wide open, I didn't think to pull it closed.

There were two more bouts of therapy following that first attempt and another few years before I dared to work below ground with Joanna, my last psychotherapist. I still played it safe, careful to measure out just how deep I was willing to go. In those easy questions like, "Tell me about your mother ..." I discovered there was an uneasy alliance between what I thought, and how I felt about her. My father too, so little known and for so short a time. I had built up a lifetime relationship with him through fantasy; he was perfect, as constant in my unconscious dreaming as he had been reportedly fickle in life. No man, certainly not the clock man, or Frank either for that matter, had any hope of living up to such perfection. No wonder I was so angry. My mother, a little wisp of a thing, clinging to her grief and disappointment like a talisman. The sorrow of her husband's death had given her definition, the pity of others a stand-in for constancy. I was the living symbol of her difficult life as a single parent whose child was doing well, my success the product of her own hard work. In those days my older brother, Tom, appeared to be running wild. If I failed too, so would she and the pity might dry up. For my mother, who depended so heavily on the attention of others, misbehaviour on my part would have killed her.