he says he never would and tbh he has never given me any reason to think he might.
He met up with her behind your back OP.
He of course tells me if she admitted it was her he would have told her to fuck off and leave him alone but I don't believe that at all.
We can see from the convo that he didn't, he knew it was her yet he carried on the conversation, at least for a while.
mostly in the dark
He sounds awful. He might well talk about you like this to other women, to get what he wants.
If you met him you would think he's such a lovely guy, very respectful and kind and nobody would ever think he was capable of treating women so badly. Its only me who knows who he really is.
If you've realized what he's like, then you know what you need to do. You are going out with someone who you're aware treats women really badly.
He says I just have to trust him
Why? He's seen her behind your back in the past, now he's been messaging with her behind your back. You don't have reason to trust him- you have reason not to.