Funny, I've been thinking about this recently (nine year saga = lots of ruminating to get through).
At the time, son was two. He'd messed me about terribly during my pregnancy - to the point where I moved to be out of the way of it, apparently attempted suicide in my third trimester and then gone straight from trying to mend bridges with me to going off with a 'proper girlfriend' when Son was two months old.
After a couple of years of being very hurt and miserable, I gradually felt in a position of relative control because I'd successfully fended off sporadic hassling, we'd straightened out some of our issues and I felt emboldened to express some of my opinions on the way that the situation had been handled (not v positive ones, incidentally). I thought we were friends. At the same time, his relationship had obviously taken a downward turn, which he kept trying to discuss with me and, again, which I kept fending off.
Then, out of the blue, he turned up at my house at 2am, very drunk, when he was supposed to be out with his gf and for some mystery reason, I decided to sleep with him, totalling his relationship and starting the whole thing up again. For a long time, I couldn't understand why, when I was in a position of relative control, I'd just decided to randomly chuck it all away and throw myself under the bus like that.
These days I'm starting to read it differently. I think that I was pretty ground down after three/four years of dealing with the same relentlessly humiliating attitude towards me and I was tired, lonely and desperate for some affection. He didn't really give me any positive attention when I 'behaved well', only when I started to react and snipe at his gf. What I thought was being in control was actually being manipulated into engaging with the situation in an unhealthy way. And that situation went to its natural conclusion.
What happened next: We were together for a month or two, he was a drunk nuisance and kept sabotaging me in weird ways, so I (nicely) broke up with him because he obviously wasn't feeling it. Then of course I got blamed for being the Evil Influence who broke up his (of course, perfect) relationship and messed about for another year or so before the devaluation began properly. I am now here instead of sipping cocktails with a millionaire husband in the Bahamas, which I suspect serves me right.
Lessons learnt: 1) Don't cheat on people (obvs) 2) Don't engage with people who periodically disturb your sleep 3) Don't pay attention to people who are disloyal or dishonest with their partners 4) Don't seek the approval of people who aren't very nice to you 5) Don't ignore your own values or break your own boundaries 6) Don't politely ignore demeaning or hurtful behaviour by others towards you because they were drunk at the time, or because they're nice to you in the meantime 7) Don't let people play you off against other people 8) Learn to fix your own vacuum cleaner so that you're not relying on unreliable messer-abouters to do it for you.