Trust?

How do you build up your trust back for a person who has completely betrayed it?

My boyfriend has done quite a few disloyal things, he hasn’t actually cheated but has been texting other girls, sexting, telling them he likes them and they’re attractive, etc. and I’ve caught him out every time and forgiven him for it, the most recent time was 2 weeks ago. I have an issue where I always feel the need to go through his phone, which is how I found out all these things. But I’ve promised him I wouldn’t do it again because it’s an invasion of privacy. But I don’t know how to build my trust back up for him without going through his phone! We just try not to talk about it and go about things as normal but I don’t feel any more trust for him. Every time I see a girls name come up on his phone my heart drops and I feel so angry and sad, even if it’s a girl I trust and know he’s not interested in. I just want to be able to move past this and carry on with a normal relationship, but I don’t know what to do to trust him again 😢

Please don’t tell me to leave him and that he’s bad etc. I’ve already made my decision to stay with him. he has been battling severe depression causing him to be confused about his feelings, but is getting better now and We both want our relationship to get better too