How to trust?

So I have this Amazing boyfriend..He’s such a hard worker he does everything he can to make me happy and get me what I need...He’s always messaging me and getting me things to make me happy but for some reason I’m so scared he’ll cheat on me..he has a past with other girls especially his Ex of 6 years who he cheated on all the time...Well I’m also scared he’s gonna leave me for her and I hate that constant sadness I feel when I think about it..he has given me reasons to think this way we moved on but I just can’t let It go no matter how hard I try..What do I do?I love him so much we’ve been together for a year and 3 months PLEASE HELP :(