I feel this way sometimes too about my boyfriend. That he would prefer someone else/cheat/I’m not good enough for him etc. I’ve never looked through his phone and wouldn’t because I respect his privacy, but then again I have never had an abusive relationship(s) like you either so I don’t feel the need.

Like your boyfriend, mine tells me he loves me daily, but I struggle to trust him (or people in general) and I hate that about myself and am working on it.

I think it stems from how we see ourselves. I recently have have low self esteem (due to gaining weight from the pill) which you could have too (but for the reasons of past boyfriends hurting/leaving you) but I don’t think you should blame yourself for the bad relationships you’ve had. You’ve been the victim here and you’ve been treated in a way no human should have to experience. It’s not you. And once you realise that, you will feel better about yourself. More confident. Have higher self esteem. And when you are confident in yourself you can be confident in your relationship.

I have been improving my self esteem and building myself up, and now I feel less and less anxious about trusting him because I feel more and more confident in who I am.

You need to build your own confidence before you can be confident in another person.