Why can't i forget the past??

I have been dating my boyfriend for 4 years. We have our arguments and disagreements but overall we are head over hills for each other. He has a drinkin problem. That cause him to cheat two years ago. He claims it only happen once so I forgave him. But it turned out he was talking to her months later after the fact. I didn't know this until a friend of ours saw him with her. He confessed they hang out but still it only happened once. I was reluctant to forgive him again but I felt like his heart was sincere. He stop talking to her and everything has been going good between us. There is this "constant voice in my head that keep reminding me of this painful time of my life. Though we are going stronger then ever now I can't seem to shake this feeling off. Sometimes I cry out of no where and I'm sick of it. I know two years is a long time to dwell on something but I can't help it. And I feel really guilty that I can't have to feeling I want to have because I'm stuck on this memory. I can't let it go and I really need help cause my heart literally aches thinking about it.
Is it normal to be this way.