Hulababy - ditto kind of. Had such a horrendous birth experience first time around that it took me three years or so to get to the point where I felt that I could cope with another child and, more to the point, wanted one. Then discovered that, in those three years, my ovaries had packed up prematurely. IVF wasn't even an option. To top it off, at around the same time everyone I knew was popping out their second or third. Cue DS wanting to know why he didn't have a brother or sister - answer: "some tummies just can't make lots of babies."
That was almost three years ago and, while I am resigned to the situation, I still have moments where I hope for a miracle. The difference is that I'm now better able to deal with those moments - by focusing on the wonderful son that I do have rather than sinking into a "what if?" spiral of depression. Think the turning point was when DS started school - really rammed home just how quickly they grow up and made me realise I didn't want to waste another minute of it mourning some, essentially, "imaginary child" and being a sad old cow when I should be making the most of every minute.
Now, when DS' school friends invariably ask if he has a brother or sister, he just explains in a matter of fact fashion that "it's just me, my mum and my dad in my family." He doesn't seem bothered by this - maybe because he does have a lot of friends (and three cousins) that we make a real effort to spend time with. He has a very definite best friend, who happens to be the son of the best(mummy)friend I made at antenatal classes, and they just adore each other (best friend talking to his mum about DS: "I love him more than sweets.") and I take solace from the fact that, while I love my three siblings dearly, my closest confidants are the aforementioned mum and two school friends - one that I made at 9 and the other at 13.
That said, yes, I'd rather he was one of two - although, somewhat selfishly, I wonder how I'd cope if anything happened to DS more than I wonder how he'd cope when DH and I hit old age.
Which doesn't really help you with your dilemma, Outdoorsmum - think the point I'm trying to make is that if I was you I'd really try to get your DH to understand where you are coming from. These feelings don't just disappear and could so easily lead to resentment unless you both really talk things through. Whatever happens though, don't lose sight of what you already have.