'Hello, I'm Zoe and I'm ... depressed." Twelve faces stare back at me, nodding sympathetically. I am in therapy for depression and I can't believe I'm here. Why am I here?

I mean, I know I suffer from depression periodically and take antidepressants, but I'm not depressed depressed. I'm just not that bad. I'm a busy woman, and to take the time to attend to some silly little person like me seems to me the height of overindulgent selfishness.

I look around the room and imagine that the other people are all here for proper depression - bipolar or unipolar or compulsive disorders that mean they can't leave the house. OK, so I get a bit down from time to time, but what's the biggie?

Well, the biggie was that my best friend decided that she had had enough of my weirdness.

"You need help," she told me, as she sat opposite me on the sofa, annoyingly entrenched.

"I'm fine."

"You are not fine," she persisted.

"OK, so I spend a lot of time alone,"

I snapped back. "I dislike the phone. I write every day. I'm driven to the point of obsession. OK, I want to be perfect - but don't we all?"

"Er, no we don't," she said, following me into the kitchen. "You walk up to eight miles a day. You take an unnaturally long time to recover from the break-up of relationships. You feel safer being alone ..."

"OK, OK, stop! I accept that I'm weird and I might need help for my weirdness but it's my fault I'm weird. It's not because I'm depressed."

"You need group therapy," she countered.

"Group what?" I shuddered at the thought.

I couldn't help feeling that sitting in a sort of "depressives anonymous" would send even the most Mary Poppins-like of us over the edge, but being an extremely nosy person, as well as a writer, I decided to go along.

A few weeks later, confronted by a roomful of expectant people, I am desperate to be anywhere else. Frankly, burning in hell seems an attractive option.

"Would you like to tell the group a little about why you are here?" asks the counsellor.

"I get a bit depressed sometimes, but it's just not that bad."

"All depression is bad," she says sympathetically. Like I deserve sympathy.

"I'm not depressed; I'm just not a very nice person. I get angry with people I don't even know - like bus drivers and people in news-agents. When anyone ever asks me how I am, I am rude because I want to cry, because how I am is bad, always bad, and they should damn well know not to ask such a stupid question, and when people call, I don't answer my phone. You see? I'm just a terrible person."

"You've just described the classic symptoms of depression," she says.

"No, no, no," I say. "You don't understand. This is what I'm like all the time."

"Well, have you ever considered that you might be depressed all the time?"

"Oh no, I've always been like this - well, since I was about eight, I suppose."

"Have you considered you might have been depressed since then?"

"No, no," I smile at her patiently. "You don't understand. I know when I'm depressed. When I'm depressed I can't leave the house. It overwhelms me like a physically sickness. It comes in waves and often I just drive around or sit in the car and cry."

"So could you possibly accept that these times you describe are serious major bouts of depression?"

I shrug and inspect my fingers, hoping that she'll move on. She doesn't.

"Do you know that a lot of people do not get out of bed for days in these bouts?"

My fingers are, at this stage, fascinating.

"The symptoms you describe are accurate for dysthymic depression - mild constant depression interrupted by occasional serious episodes."

After that first session, I take a walk. It has become stunningly obvious that my noticeable traits of character - things I had put down to personal idiosyncrasies - are actually symptoms of depression: isolation, difficulty having relationships, feeling sad, rejecting affection.

I will save you the sob story, but my parents were unhappy together and finally divorced when I was eight. My childhood losses and my feelings of rejection and abandonment could have kick-started the depression.

From both my own experience and what I have learned from psychiatrists, depression is the same feeling as grief. Grief occurs naturally after a major trauma such as an accident, bereavement or physical or emotional abuse. A person can be genetically predisposed to depression (ie, the body doesn't produce enough serotonin or some other "happy chemical"), but there is also a medical theory, to which I subscribe (promoted by Andrew Solomon in The Noonday Demon) that the brain, being a clever little sod, can learn to grieve all by itself. It says that lots of little traumas happening regularly throughout childhood and beyond (such as abandonment or the departure of a parent, or bullying or rejection) mean that the brain starts to recognise the feelings associated with grief and starts to repeat them whenever it feels like it, in the end not needing any traumatic event to stimulate it. Hence one starts to feel sadness and loss seemingly for no reason.

The knowledge that I am suffering from a depressive illness, coupled with my ability to discuss it with others in the same position, is nothing short of an epiphany. As the other people in the group talk, I find myself thinking, "My God, that's me - that's what I do" over and over again. Suddenly seeing that I am not alone in my weirdness is liberating. Perhaps I am not so terrible after all.

Then the group gives me feedback.

"I see a woman who is very hard on herself. Super-critical with impossibly high standards that she can never meet," says one man.

"I see a woman who is sad and needs to show herself a little compassion," says a lady with bipolar disorder.

Compassion? Me? Ha! That's a load of bollocks. I don't need compassion. I just need to do more, work harder - compassion is for other people.

My inner critic rages. Up to now I have relied on it to urge me forward, to keep me driven. Now I find that my negative voice is a manifestation of my illness, the lack of serotonin asserting itself, telling me I am bad and worthless.

"Are you ready to give yourself a break now?" The therapist smiles kindly. "It's been 28 years - you deserve a bit of peace."

I bite my lip and nod. Suddenly I feel like an eight-year-old again. I am made to write a letter from the adult me to the eight-year-old me, telling her that I love her.

I feel more than a little uncomfortable loving myself. I'm so used to hating myself.

You see, all those years, I thought it was normal to wake up in the morning and just see grey pointlessness. I thought it was normal to cry whenever you heard music. I thought it was normal to stare at people in the street and feel overwhelmed by sadness and futility at the sight of their pointless lives. I thought it was normal to avoid my friends most of the time. I've been seeing life in grey - like a rainy day in winter - and thinking this is what everybody sees. Happiness, whenever it has come, has been fleeting and so unfamiliar that it makes me sick with anxiety.

All of a sudden I can see that I have an illness that has gone untreated - perhaps dangerously so, because there are times I have felt so destructive or despairing that I have had no regard for my health. Now I am getting the help I need.

Of course, I am not the only person who has difficulty accepting this nebulous illness. Many people do not believe depression is real. There is no blood test that proves it, no physical handicap. Unless someone is crying in front of you how, can you tell they are depressed? They are probably just making it up, trying to get sympathy, attention, a day off work ... But let's turn this round. Why would someone want to be unable to get out of bed or go to the shops or talk on the phone or raise a smile?

I'm just thankful for the friend who loved me more than I loved myself, who stepped outside of her - and my - comfort zone and pointed it out. If you think you are depressed, you probably are - and there is no reason you should miss out on the everyday happiness that most people take for granted.

· Zoe Lewis's new play Oyster is at the Hen & Chickens Theatre, London N1 (020-7704 2001), at 7pm tonight.

· Do you have a story to tell about your life? Email it, including a phone number if possible (no attachments, please), to my.story@theguardian.com