Dealing with past hurt

I'm looking for an effective way of letting go of unresolved past trauma.

To cut a long story very short, my boyfriend slept with his ex wife during the very early stages of our relationship and told me about it a year later. I chose to stay and work through the inevitable trust and self esteem issues that followed. Things were extremely difficult at first but got gradually better and a few years later we were stronger than I thought possible. My trust for him returned and then some, and I genuinely felt at peace with what had happened. I accepted and understood why he had been unfaithful and knew in my heart that he would never hurt me again. He was a different man to the one who betrayed me and still is. We are still in daily contact with his ex as they have a child together which made things in unbearable at times as to cut her out for good would have been a better option for me. If there's ever a disagreement between her and my partner she will bring up the past in an attempt to hurt us, which I must admit does work sometimes.

The problem is that lately I have been thinking about this alot. Feelings that I though we're dealt with have surfaced again for reasons I cant fathom and I'm finding myself upset daily about what he did. Thoughts of the two of them together will suddenly enter my head and make me want to throw up and I often find myself going over conversations that I've seen between them or things that she's said, driving myself crazy wondering if there was more to it.

I've talked to my boyfriend about this, and he tries to be reassuring but is obviously upset if I bring it up as breaking my heart isn't something he's proud of and hearing how I'm still hurting makes him feel disgusted with himself.

I don't want to leave. I love this man and I know we are bigger than things from our past. I know we are stronger than our darkest moments. We have a future together and I am happy with him. I don't understand why this has suddenly become an issue for me again- he hasn't given me any reason to doubt him since. Hpe can I let go of this for good and just move on? :(