At last, some time to reply.
I've been thinking about your replies and your ideas all day. What a day for navel gazing it's been - I've irritated myself to death, but it's been so very useful. It's probably going to irritate you, too.
Re whether DP was being manipulative - actually, no. Like a couple of you said, this came out very early on (like, second date??), in a jokey kind of way. I asked him a few weeks later on why he said it and he said it was tactless, he'd immediately regretted it and he was probably sort of trying to establish himself as someone who had sex (and wasn't just a sap sending me poetry). I guess that is a crass kind of manipulation, but, really, it's not what he's like. BUT - I definitely HAVE been subject to this before (DH was a sod for it) and I think I spent a long time forming these opinions. DP's comment was an unfortunate trigger. He certainly hasn't repeated it.
To continue on from the fact that I have developed this tendency to jealousy over a long time: god, that is a whole world of woe, which has pre-occupied me all day. Some of it is around the experience of being cheated on, some of it comes from my own past as a bit of a cheater (not when I was married - but, in my teens and 20s, I would end relationships by finding a new one), but the bulk of it comes from a general inability to picture myself being content.
DP and his ex-wife were not very content: they had great times and they had terrible times, but it was always pretty rocky. Something about that seems more 'real' than our easy, companionable, mutually supportive, loving, light-hearted relationship. I feel like some dull aftershow, whilst also knowing that this is bonkers. I love our relationship - it's all I've ever wanted. Except I think I might be a drama addict ... somethiing about adversity compels me. The fact I stayed with DH for so long is evidence of this; we had a horribly adversarial relationship. So I am feeling this nice, calm contentment and thinking, "WHAT IS WRONG???? WHERE IS THE DRAMA??? THIS IS DULL AND MUST MEAN I AM IN A RUT!".
I know this is shit and must seem very facile, but it has only just occurred to me. I've spent my life going after unavailable or difficult men. I made a conscious decision (pre DP) to be alone rather than repeat history, and that was going very well. DP seemed perfect (he is great for me - we have the best time together). I am worried I'm going to mess it all up by creating drama. God, it's just so pointless.
Finally, re competitiveness and the ex-wife: actually, I got past that pretty quickly. It's not so much her - she has good and bad points, like the rest of us and I'm not really very interested in her life, as long as she doesn't cause us harm. But it is about his life with her. I am jealous of that. They had 15 years ... that is an Everest and we are still at base camp.
I'm afraid it will fail. I'm afraid I am incapable of a functional relationship.
Tedious, sorry. Thank you all for your replies; they have led me to some very unexpected perspectives, which is proving to be well overdue.