Hello, I'd like to help you with your question.
It sounds like you and your boyfriend are working hard to make your relationship better. And trust is a big part of being able to do that. Your boyfriend seems to feel it is important to address all the issues before you both take a big step in your relationship and get married. It sounds like he does love you very much.
While your feelings about the other person are understandable, you seemed to have moved on from the relationship overall. The feelings you had for the other man were not enough to base a good solid relationship on. That may be one way to present your feelings to your boyfriend. Let him know that while you did
have this relationship with the other man, never did you consider the relationship any more than just a distraction. If you feel you need to tell him about the type of feelings you had for the other man, add
that these feelings were about you and not about the man. Let him know these are feelings you need to work on yourself so you can let go. Then talk to him about how you intend on addressing these feelings.
The best way to address what you feel for the other man is to change how you think about it. When you feel something that strong about someone, it can feel overwhelming and uncontrollable. But if you change your thoughts, you can alter your feelings. So for example, when you think about this man and feel attracted to what you had together, alter your thoughts by saying to yourself "yes, those are my feelings but they are just feelings. I don't want to think about this right now" then distract yourself. Also, add that you do not have to listen to your feelings about him but pay more attention to your feelings for your boyfriend. If you feel that you are unable to change your thoughts about this man, you may want to consider talking to a counselor about it to help you find new ways to leave the relationship behind and to find out why you feel as you do. There may be something missing for you that you need to address so you can move on. Plus if you do seek therapy, your boyfriend will know you are serious about changing things and improving your relationship. That will help him learn to trust.
Also, tell your boyfriend that you always felt that the relationship between you was the one that was most fulfilling to you and that you want to try to work on ways that you can get more excitement between you. It may not reach the level you had with the other man (being with someone outside of your relationship always brings a level of excitement that is hard to match in normal relationships) but you can make things better. Read books, seek out new ideas and be willing to experiment with each other. Going outside of your comfort zone can make things seem much more interesting.
I hope this has helped you,