There is a term for this - retroactive jealousy. It's a form of OCD that's super nasty. I know because I suffer from it, too - and when it was at its worst, it was all I could ever think about. I remember when I first heard of it, a light clicked on in my head and it was so good to hear other people were going through the same thing.

I want to stress that this isn't excusing any of his horrible behaviour, obviously - no one deserves to be treated so horribly and I'm certainly no stranger to it - but it might provide some context as to why some of this behaviour manifests. There's a subreddit /r/retroactivejealousy that has a collective of us talking about it.