Is it strange that I am usually having no opinion strong enough to do anything other than being "ok" with going along with what everyone else wants? I mean, I know what I want and what I like, but I feel so guilty and anxious trying to get it when other people start making demands so I try to alleviate my anxiety by just saying sure, we can go to eat there, I am fine with those curtains, that movie looks alright to watch. And I am really thinking, I kind of wanted chinese this time, omg I hate orange curtains, I don't like war movies bleh.
Well, you most probably do have opinions strong enough. . .you just sweep those aside for the reasons you have identified - which by the way is a massive step

I can be a bit like this - I called it 'easy-going'. . .which sometimes it is, sometimes not. I think back and there were lots of things with my ex I would go along with - nothing that impacted majorally on me until the end. Then I knew I had to stand my ground or he would have taken advantage of me forever. . .he was clearly testing what I would put up with. Cheating, lying, emotional abuse and manipulation was not something I would allow. . .although I was like you - desperate inside for him to stop it/change it/realise it. In the end, I had to manage this myself.
Probably I seem to many people here like I am BPD myself
No, you are just confused. . .and then you cannot think clearly. Everybody here has most probably questioned themselves
He reacted, first with anger, then silence, then confusion... . he says he doesn't even know what to do now. He is baffled. He doesn't get it. All he says he gets is that I am upset over her but he isn't doing anything wrong, he wants things to be ok, and so he gets frustrated and starts trying to MAKE me be ok because he is tired of it. He is really frustrated.
I expect so. It's not going to (his) plan is it? He wants you to stop putting up boundaries, let him get his own way and sweep it under the carpet. . .again - with the rest of the rubbish he has never taken accountability or showed any empathy for.
He doesn't understand why after all this time, I am in such a state of grief. He doesn't appear to comprehend the long term emotional effects of his actions. He said he felt like trashing his accounts of his game was the only real solution. I don't need him to trash his accounts, I need empathy and for him to stop trying to force me to "get over it" on his time and instead let me heal on *my* time
We're back to he won't. . .that's down to you. . .and it's up to him what he does with his 'game accounts'.