rhubarb, since I read your post earlier I have been thinking about you. I know you came out of a very bad time and wanted to share the experience to help others - I looked at your site a while back.
I never suffered from that type of deep depression when I was pregnant, but some of what you went through rang a small bell for me.
I have a big non maternal streak running through me. Even in my twenties I never wanted to get married or have children and felt really sorry for those of my friends who had settled down. I saw my grotty flatshare, my lack of car, cash, steady boyfriend and serious job as great advantages compared to the shackles of husband and proper mortgaged home. They had babies, I had evening classes - and I knew which one I preferred. This feeling subsided a little in my thirties but was still there and, as I've said on other threads, I became pregnant with reluctance and pessimism. I was missing my freedom even before it was taken away from me.
My pregnancy was 'easy', so was my baby son. But both dh and I decided to stop at one - especially when my son became a demanding litte toddler.
However, as I approached 40 I had second thoughts - thinking of our small family and how my son would be the only one remaining when dh and I died. Also I didn't want ds to be the focus of all his parents and grandparents attention - too heavy a burden. And, well, I found I actually liked being a mother. My dh was not at all sure about me doing this U turn, but he realised I wanted to try very much, so he said OK. We decided to give outselves six months to try - leave it in the hands of fate, give ourselves a last chance - then it nothing happened just get on with our lives. As the end of the six months approached I happily reconciled myself to having no more babies. I felt totally at peace with the future, a future with one child. Thought about getting my coil fitted again. We had given things a chance. Then at the end of the sixth month my period didn't happen. I was shocked, felt no this can't be happenning to me, not now please. I saw my lovely one-child future slip away. I blamed myself, felt really low. Remembered the saying, 'be careful what you wish for, because you might just get it.' I felt fate had played a joke with me and I was not amused. Kept waking up and hoping it was a dream. And what's more because I was the one who had pushed for a second, I had to put on a front, be positive and happy about being pregnant again, make light of my worries to friends.
I did lots of hard thinking about my life and decided I had to change other things in it, in order to feel happy about this pregnancy. I deliberately used the pregnancy to force through big changes. I persuaded my dh that we had to move to release capital in our house so that I would be able to take a good maternity break from my full time job, and stop running my market stall. In the meantime, I passed more of the market stall work over to my dh. Admittedly my dh didn't need much persuading over the move because our house was so small, but whereas moving had been an idle thought on his part, me being pregnant put a deadline on it. So we got out the paint pots, mended and decorated, and put the house on the market. In other smaller ways I began to take a back seat in the family - I talked to dh and we approached his parents, asking if our older son, then aged five, could stay with them while I was in hospital. They were reluctant at first, thought he was too young to stay hundreds of miles away from his parents - but I said how difficult I would find it having to cope with both baby and son, so they agreed and my son had a lovely holiday with them.
When my second son was born, he was so beautiful to me. He felt like a special gift. I spent the days in the maternity ward staring into his eyes and answering calls from the estate agent who was selling our house. Looking after son number two was a doddle compared to son number one - I only had to relearn things, not learn them from scratch. I knew far better what was serious and what was not. It is so stressful having to feel your way all the time and such a relief when you don't have to. Familiarity made a huge difference and I really second Monkey's post about two being easier than one.
And all these changes were for the better, and I felt my pregnancy had made them happen quicker. It gave me a sense of control in my life, against the shock news of the pregnancy. This helped me feel more positive about things and look more happily to the future.
Everyone has their own story, rhubarb, and since I haven't suffered with pre or post natal depression, I won't presume to know what you are going through or whether making changes in your life is appropriate or possible right now. But I wanted to tell you my story in case it some small way it helps you, as I know your story has helped others.