The train picks up speed as it heads out of Leicester and I stare out the window as the countryside flickers past in varying shades of January grey. I'm thinking I should have got the earlier train because this one is running late; I'm thinking I wouldn't want to live round here. Then I'm thinking I don't really want to live that much at all. It's not that I actively want to kill myself; it's more that being dead doesn't sound too bad an option. I start to wonder what it must be like to be the only person pleased to be in a train crash. This wastes a good 15 minutes or so before we pull into Loughborough, where I get off and carry on with my life.

There's nothing particularly unusual about this. I've been depressed on and off for years and years and this current episode/bout - I'm not really sure what the proper term is - only registers about halfway up the bleakness scale. It's more than a touch of the winter blues - my sleeping is messed up, I'm anxious and it's a struggle to leave the house in the morning - but it's a long way off from the panic attacks and uncontrollable catatonia that have seen me off work or institutionalised for weeks on end in the past. If I'm careful to plan my life properly, if I can bring myself to let family, friends and colleagues know what's going on - not too much detail, mind; I don't want them all to do a runner - and if the medication does its job, I'll get through this relatively unscathed in a month or so.

Not that it's going to be much fun. I hate waking up in the morning and feeling as if I'm not really here, as if there's nothing inside me. I hate feeling that my identity has dissolved and I'm giving a half-remembered performance of myself. I hate the crushing sense of failure that makes everything seem equally futile. I hate going through the process of telling myself I've got a lovely family, great friends, decent job, blah, blah, blah, because I know it's completely pointless as I'm not really here anyway. I hate the way this disease is just so fucking relentless and I'm tired of having to battle my psyche year in, year out just to keep my head above water. Above all, I hate the effect it has on all my relationships.

I wasn't exactly a picture of pristine mental health when I got married, but the depression didn't really kick in until some time later. Not that either me or my wife knew enough about the condition to recognise it for what it was at the beginning.

It started as a series of hypochondriacal illnesses that were accompanied by a crippling sense of anxiety and only went away after countless visits to the doctor and a barrage of unnecessary medical tests had eliminated whatever fatal disease I happened to think I was suffering from. The intervals between the various cancers - I've had them all, even breast cancer - rapidly got shorter and shorter and the intensity of the anxiety developed into prolonged panic attacks. At times, I could barely leave my bedroom, let alone the house. I couldn't sleep, I couldn't talk, I couldn't read, I couldn't watch TV. I just was.

The crunch came at 6.30am one Christmas morning. I could hear the kids getting ready to come into our room to open their stockings and I leant over to my wife to say, "I think I've got BSE." Her response was to start hitting me. It didn't hurt and we laugh about it now - well, I do - but it wasn't very funny at the time. It shocked us both because it was just so out of character. My illness had turned a normally well-balanced woman violent; she had morphed from compassion and understanding to incomprehension, fear and complete frustration.

Somehow we navigated the rest of Christmas, and in the new year we sought help; within a day of seeing a psychiatrist I was tucked up in a mental hospital being treated for severe clinical depression.

Naively, we both rather hoped this was a one-off; that I would come home sorted. It hasn't really turned out that way. There have been periods of time - years even - when I've been OK, but the depression has always come back at some point. It's never been as tough to deal with as the first time - familiarity does have its advantages - but it's still a complete bitch because the process is always the same. I go from being a person to a non-person. If anything, though, it's worse for my wife. She loses a partner and gains a child.

Everything changes with depression. All the fixed points of the day come up for grabs. I can barely look after myself, let alone the kids, so everything that was previously shared becomes her responsibility. Who's going to get the kids up, take them to school and be there for them in the evening? Is she going to have to take a day off work? Every decision is down to her, because even if I'm there, I'm not there. And that's just the practicalities. It's my emotional absence that affects her the most, because my isolation imposes its own isolation on her. She can't talk about how difficult she's finding it all as you can't have that kind of conversation with a kid.

And here's where everything becomes murkier still. Once a fixed point has proved to be negotiable a couple of times, it can't really be said to be fixed any more. So even when I'm not depressed, the threat of depression manages to leak its poison. How do you make arrangements if you don't know quite what's going to happen? Is it worth booking that half-term holiday if I'm going to make it miserable for everyone? Who knows? We don't. So far we've just said sod it and gone for it, but as the number of holidays I've managed to ruin starts to climb still higher maybe we'll decide not to bother one day.

Dealing with the emotional fallout is just as tough. How can my wife know if she can always rely on me for support? She can't. She has to hope and trust. I feel just as crap about this because deep down I know she can't always count on me because there will always be times when I flake out.

This sets off an unpleasant little dynamic of its own. You see, I know exactly how my wife is going to react to my depression. She'll start off compassionate, but as it goes on she'll get more and more freaked out because, much as one would want it otherwise, depression is - at a primal level - incomprehensible to the mentally well. And when it's all over and I'm back to normal - ish - and it's safe for her to express her feelings, she'll be furiously angry with me for a couple of weeks for putting her through hell. It's all par for the course: she's not a bitch - I'd be exactly the same if the roles were reversed. But it is hard going.

So when I can feel a depression coming on, I've learned to be a little cagey with my own feelings and don't always tell her how bad things are. I say things like, "I'm having a tricky patch" and hope that by gently raising the issue I can stave off a more serious depression and her chain reaction. Most of all, I try to avoid letting her look into my eyes in case she notices the emptiness. It's a difficult balancing act. Sometimes I get it right, and sometimes I get it wrong. But even when I get it right, I don't get away with it completely. Hiding emotion is toxic to intimacy.

We get by, though. In fact, often we do rather better. There's nothing like a crisis to cut through the everyday crap that wears down every relationship, and the good times are, well, good. But then, for the most part, we are adults, and during the times when I'm well we can talk about it. Mental illness is not a taboo: we get angry about it, we argue about it and we laugh about it - even if my wife still doesn't find death jokes quite as funny as I do.

The kids are another matter. My daughter is now a teenager and was only a toddler when my depression was first diagnosed; my son was just a baby. My wife and I have never tried to pretend everything was OK when it wasn't; but equally, you don't want to go round frightening kids by giving them more information than they can handle. So whenever I've been severely depressed, we've tried to treat it as an everyday part of family life; everyone's a mess from time to time, it just happens that round here it's generally Daddy, and if there's anything they want to know, they should ask.

Mostly they appear OK with this. When I'm in a state they seem to intuitively understand that I haven't got anything to offer. They'll come and give me a cuddle, check to see if I'm OK and then bugger off to annoy their mother or their friends. So it's tempting to think they're normal and that they've got away unscathed. But it's obviously wishful thinking. How can there be no fallout when you've visited your father in a mental hospital; when you've seen him lying for days on end on the sofa saying next to nothing, crying intermittently; when you've seen your mother worried sick at how she's going to cope?

Some of the fallout may be good. They may have a healthier attitude towards mental illness and be less afraid of expressing difficult feelings than other kids their age. They've certainly got a better sense of humour than any of their mates. But as I'm a depressive, I prefer to dwell on the negatives. I worry they've had to grow up too fast, that they've learned the world is a scary, unsafe place and that they've inherited my own sense of impermanence.

Only a few weeks ago, my son said to my wife, "I'm worried about Daddy. Is he going to commit suicide?" That's no kind of question for an 11-year-old to be asking. We tried to reassure him and he appears to be over it; but who knows what's really going on in his unconscious? And, years down the line, how much will it cost him in therapy trying to find out?

I should probably learn to worry less. What's done is done and the kids will sort things out for themselves one way or another. But as anxiety is hot-wired into my psyche that's a pointless suggestion. I doubt I'll ever stop wondering how things might have turned out. And what's the truthful answer to my son's question? Am I going to kill myself? Nah. But I'll probably still think about it from time to time.

· Will Keighley is a pseudonym