I feel special.  Most women will have only one menopause, and they will hate it. I will have two, and when the second one comes, I will know what is coming.

I am having my extra menopause as a cure. I have stage 4 endometriosis. Lots of women have endometriosis - 22 out of 100 in the UK; ten million in the US - but even more women have never heard of it, though some of them undoubtedly have it. I didn't know I had it until I began to get bloated. My mother, because she is a mother, thought it was ovarian cancer. I thought it was a suddenly unsuitable diet. My diagnosis took a year to come, and not before I'd spent £70 on a kinesiology test from a woman from Huddersfield whose other job was providing hog roasts for weddings. I want that £70 back, because there is no way I'm allergic to coriander, and she didn't mention that I am riddled and diseased, with her finger tapping and strange metal objects.

I also started NHS investigations: two ultrasounds, two times they found the same cyst on my ovary, which grew no bigger after several weeks. That was a good sign. Then a diagnostic laparoscopy, for which I had to travel to the other side of my car park, having the fortune to live next to a hospital (not so good when the ambulance helicopter does landing practice). In the recovery room afterwards, the consultant came in and said, it's good news. It's not cancer. But it is stage 4 endometriosis. I'm not sure where I found the presence of mind or even any mind at all, when it was whoozing round the room, but I said, 'how many stages are there?' Oh. He smiled laconically (he's Edinburgh and laconic). "Four." Later, he had a much better description of it: "Your abdomen," he said, "is a fiery furnace."

I am riddled. Diseased and ravaged; strafed and clawed; ransacked by my own body. Every month, for at least ten years, the endometrial lining of my womb has, rather than be totally discharged as a period, gone for a wander around the rest of my body. It has attached itself to my bowel. It has glued together my uterus and rectum. It is all over my pelvis. And it has placed a cyst - the chocolate kind, as endometrial cysts are called, sweetly - on each of my ovaries. It has probably - there is a 40% chance - made me infertile. It is incurable, but it is not terminal. Only removing all my reproductive equipment - a procedure known delightfully as "a pelvic clearance" - will cure it. If that doesn't sound bad enough, consider this: Every month when I get my period, every single scrap of endometriosis bleeds in sympathy. Every month, I am a cacophony, a symphony of bleeding.

So I have my menopause. Really. The drug is called Prostap or leuprorelin (a word that neither I nor the hospital pharmacist could pronounce with ease). It is a gonadorelin (LHRH) analogue which switches off my oestrogen production. The way it does this is slightly odd: first it overloads my pituitary gland with the hormones that create oestrogen, then my pituitary gland thinks it needn't bother making any more, and then the drug-supplied oestrogen runs out, my ovaries switch off and there you are: I'm 41 years old and menopausal.

That I do know. I know that they think switching off oestrogen will calm the inflammation and the fiery furnace. But the rest of it, the destabilising, depressing, enraging side-effects? A medical mystery. It is unsettling how often you hear doctors say "we don't really know", because they have so far given that response to: what causes endometriosis; why oestrogen-blocking, menopause-inducing drugs seem to reduce its inflammation; and why all the horrible side-effects of the menopause drugs happen.

The hot flushes, that make me feel desperate to run into my freezer for several minutes at a time, dozens of times a day, that make me constantly smell, that defeat the strongest deodorant, that make people laugh and smile when they see sweat pouring down my face, again and again? What causes them? They don't know.

The insomnia and night sweats so that when I'm not awake because I'm kicking off the bedclothes, I'm awake because I'm cold and covered in cold sweat and putting them on again, and other than that, I'm awake, my eyes closed but so wide awake behind my eyelids, they feel propped with needles? What causes it? They don't know.

And the libido-destruction. Oh yes. I used to have a sex life. Actually they do know what causes that. The pesky oestrogen-tampering. They just forgot to tell me how soul-changing it would be, how my sexual soul would be frozen, how I would become ice.

How can I describe my asexual nature? Sex is a distant memory, like an unreachable shimmer on a horizon. The trouble is, destroying your libido means you don't want to reach that shimmer. Suddenly sex seems like a foreign language, spoken by foreign people, and one that you have never spoken, for all your body reacts to anything.

I feel de-animated, passionless, desireless. Kissing is for greeting only. When my monthly injections wear off in the fourth week, I get a flicker, and I remember it, but far-off and cloudy. But mostly I am frozen. It's like I'm 85 years old (though not all 85 year-olds), or a child. All my adultness is gone, all those fertile, romping middle years of life. This must be what castration feels like.

But I am lucky. Endometriosis can cause awful pain, and I have never had that, beyond the two painkiller-dependent days a month I thought were normal. (They are not.) The treatment is finite (three more months to go); and the menopausal effects supposedly reversible. But I feel messed with. I am a woman being experimented on by men (my consultant is male, of course: the nurses are women. They give different advice, consistently).

A friend who took a libido-smashing drug too said, "I felt liberated. Like I didn't have to make all that effort any more." I don't feel liberated. I feel like a fiery furnace, but also like ice. Sleep-deprived, sweating ice.