I'm trying my hardest, really. Have no difficulty with XH being in a new relationship. He is a grumpy and difficult man at the best of times, but has been in a far better mood since he's got a girlfriend. He did all the right things: discussed with me first before introducing her to the kids, only introduced her after they'd been together for ages, introduced her as 'friend' first, etc. Now it's all official and the kids know she's Dad's girlfriend. It grates, ever so slightly, that she is buying them expensive gifts and is now going to be spending Christmas with them (my teen DD is not happy about this but is putting on a brave face for his sake, she says). But, apart from the welfare and happiness of my kids, I believe the rest is all really none of my business.
It's the history that is the most interesting bit though: He had an affair with her (yes, the same woman) when he was with first wife (25+ years ago), then dumped her. Then got back together with her, and then dumped her again (in a not very nice way) when he met had an affair with me and I became his second wife. AND NOW THEY'RE BACK TOGETHER AGAIN!
When we were together, he completely character-assassinated her, I suppose to justify leaving her but of course I believed him at the time . He told me all sorts of awful personal things about her, her past, her family, etc. Things I really wish I didn't know. Over the years he never had one decent thing to say about her. I feel quite sorry for her that she has taken him back a third time. She's not a bad person and I know her to be kind, etc.
What does bother me though is this: all the awful things he said about her, I strongly suspect he is now saying about me (to her, of course!). I was the one who left him (EA relationship, NPD, it was bad...long story) and he was terribly bitter about it all. He didn't speak to me for nearly two years after the break-up. And I mean literally not at all, wouldn't even make eye contact when collecting the kids.
I know there is absolutely nothing I can do but I'm just feeling a bit overwhelmed by it all at the moment, especially the plans for Christmas.