trust issues...

How do I get over these trust issues with my SO? We've been seeing each other for 3 years. The first two were rocky. It was off and on, he was always Taking to another girl or would sleep with another girl, so I started doing back what he was doing to me. But we'd always be back together. It was like he wanted me but he was scared to commit and get hurt, so he would do what he did to try to push me away. But I couldn't let go, he neither could he. We later ended up being together officially and grew up from doing those things to each other for the most part. Once in a while a girl would come up because he would be looking them(certain girls he messed with when he was with me) up on Facebook it caused a few fights but that was as far as it went it happend twice the past year, but we mostly I just let it go since that's all it was. And since we officially were together he swears up and down he hasn't done or has anything to do with another girl besieds me. And I somewhat believed him. But I'd still go through his phone and stuff, he ended up putting a passcode because he was tired of me not trusting him when he wasn't doing anything. And because he doesn't do that to me because he trust me. And I never found anything in his phone besides those few times he looked up those girls. He's changed a lot to where I should trust him because I never found anything in his phone as I said(I'm FBI when it comes to that stuff I'll figure out how to find something), I always know where he is, he's had me move in with him, he wants to start ttc. He's just changed a lot all for me. And I feel bad I still just can't trust him 100%... It's just because of the past. I'm stuck in that because he hurt me and broke my trust so much then. I have trust issues with anyone really. Because I know how people are, I know how I have been to people. So it's hard. I don't know how to ever trust him like I should. And I feel so bad because it's like I know he hasn't done anything bad to me over the year and that he's changed so much for me in many ways. I just don't know what to do😔 I hate feeling the way I do about it, and it's hard to talk about because I know he's just going to get mad that I don't trust him at this point. How did you get over your trust issues?