The oldest man on my street, Eric, has lost his keys for the umpteenth time this week. In the time that he repeats the routine of shuffling slowly from the front door to his car, opening the passenger door, turning on the light, looking through his key wallet under the light, unclipping a random key and then walking slowly back to his front door to see if it works, day has become night.

"Need any help?" I shout from across the street, because Eric is deaf in one ear. "No, no," he replies cheerfully, but I cross the road anyway. I always look for physical signs of degeneration, such as dirty clothes, fingernails, or a strong smell of urine that would suggest he hasn't been washing. But he is immaculate as ever, his shirt ironed, cuffs and collar clean. When we find the key that works for his door, I notice that the hall of his house is as tidy as an officer's mess. The only thing that is falling apart is his mind, and the speed at which it is happening is alarming.

Up until a few weeks ago Eric had neighbours, Stan and Eva. They were in the very old-age stage of life together. Eric would drive 200 yards to the shop to collect their newspapers if if it was raining or, if dry, he would walk with his stick. Stan died a couple of weeks ago and Eva has been taken into residential care so everything in Eric's life has changed. And quickly.

Call this the beginning of dementia. Call it immense grief. Call it loneliness. It is certainly not craziness. Eric is in his late 80s, and in the few years I've lived on the street with him he has never been anything other than totally with-it. In old age, the reasons for losing one's mind can be a mixture of all these things and it is hardly surprising, but nearly always upsetting.

I think I've lost my keys nearly every time I get to my front door, but I am not old. I call my children by the cat's name, my brothers' names. But this is just confusion, tiredness.

The mental deterioration that I feel comes from a desire to try to control the way in which my brain actually works. I have always been chaotic, always scatty. Seldom am I wholly present in a room. Yes, I can do things to improve the ways in which I function, but like somebody who is naturally highly organised, I have to understand that there is only so much of one's personality that can be changed. Yet I still have regular fights with my character traits: every day, all the way.

Take lost keys: I berate myself for being such a scatterbrain when I really do lose my keys, ie when they are not lying in the bottom of my bag along with wrapperless tampons and a half-eaten sandwich. I think about these keys being picked up by an opportunistic passerby who might try to break in to the house and steal more keys to precious things, keys that lie in a bowl right by the front door, like metal soup.

But, slowly, I'm letting myself off the hook. I'm admitting that everyone at some point loses their keys. I am trying not to see this as a unique problem associated with people like me, who find high stress bearable, but the chaos of day-to-day life difficult to deal with.

I've always had strange, illogical ways of thinking and often use those strange, illogical ways to try to sort out my problems. For example, when I was nine, I got worms. My mother took me to the doctor, and I remember thinking he looked like he had been taken down from a shelf and dusted off. He scared me before he even spoke and it was an experience that still makes me shudder.

When the worms returned, I thought I was abnormal, a serial catcher of bad things, and I feared being sent back to that old GP with the cold fingers. So I told no one, and made a vow to myself to wait until I was 11, when I'd be allowed to take a bus all by myself to the chemist in town to buy some tablets (we lived in a village with only a post office and a pub).

Two years. I would rather have waited two years with an itchy bottom and an insatiable appetite, than tell my mother. The worms left of their own accord, but my twisted logic did not. It is still here, but now I can see it for what it is and ask for help when I can't cope on my own. It can't change personalities or behaviour, but it gives me a certain empathy for and understanding of others.