Lazycow - I really identify with what you say. Ten years ago I was in the same position - no family anywhere near, 41 years old, exhausted, suffering from PND. I also had a bad back so I couldn't walk from the pain of sciatica a month before due date, because of ligaments that slackened too much. Was told I had a high chance of having the same again with a second child. Knew I couldn't put dh, dd and myself through PND again. Also, If my back had been really bad how would I have coped with a toddler and baby with no help?
Went to the NCT to hear some woman going on about spoiled only children and how everyone should have two, knowing even then when my dd was a baby, that I wouldn't be having any more. You stick to what you feel you can cope with - remember none of those righteous, interfering types will be up at 3 a.m. to look after your sick child - I bet most of them have loads of family support. I had one acquaintance who did nothing but moan and her mum and mum-in-law lived in the same town !!
I am very close to dd but felt I should be at home more for her so have only worked part-time from home. It was more difficult as she got older (now 10) when she felt lonely or wanted brothers and sisters. I tried to point out pros and cons and just be there for her. Most of the time she is happy with plenty of friends and no conflict because she can go to any activities she wants to. I still know I did the right thing. If I had been ill she would have suffered as well as the rest of us. Have told her I had a bad back so far - will tell her truth when old enough to understand.
Us parents of onlies should stick together. Nearly all of us made the decision because of health problems - I think it is a responsible one to make. If I had been younger and fitter I would have had another for my dd's sake, but it just wasn't an option. Do what you feel is right and make the best you can of all of it. I have enjoyed not having to divide my time, love, money and attention, I must say.
Hope this helps.