Hello everyone

Just facing one problem in personal life, ay be someone had similar problem.

I'm dating very very nice guy, we are planning to get married, i'm very happy.
One problem: he know my ex-guy (it wasn't real relationship, it was lets say just for health, i thought i could have such relationship in my past, but i was really mistaken and I regret it 100 times, but it is there, in my past, i would love to erase it from my and his memory) and sorry for this intimate detailes, but i have to say that which is not easy for me..
So - when we are making love, he said that he visualizing me with that guy, my face, poses we are using, my behavior etc.. Actually it's happening after we are done with sex, i mean we don't have problems with sex at all - satisfied and happy.
And i really love my boyfriend, he is everything to me, i never been such happy as i am now.
And I see he is suffering from this situation, he doesn't want to remember, but it's just jumping to his head as he is saying.
He told me about it today, he was trying to keep it with him, don't upset me, but today he said that he needed to talk with someone and he spoke with me not to blame me, just he saw only friend in me he can talk about it.
I don't know how to help him. Did anyone get out of this kind of situation?