I need help

Ok so me and my boyfriend has been together for almost 3 years now, and I love him to death and he loves me but the only issue I have is that I just can't trust him. He's messed up a lot in the past but I just can't let the past go. I feel even worse because I told him I would trust him again, at the time I fealt like I was ready but I keep holding on to the past and that's what is stopping me from trusting him again. I absolutely don't know what to do it's driving me crazy. I want to move forward from it because it happened 2 years ago so I don't know 😕