My GF cheated on me a year ago and i think im not over it.....?

weved been together 15 months.. after that infidelity in the cruise a year ago and she said she was so drunk and for a year she was nothing but nice, honest and i really think she wouldnt do it again. but i dont know why i get reminded of it everytime and i cant move on. i love her to death. and i know she does to. but everytime we fight i keep bringin it up over and over again. i know forgiving her means moving on and not dwell on the past. i dont know what to do. i love her she loves me. but damn... why am i behaving like this. im tired of tormenting her and scolding to her all the time about this. i cant move on for some reason... anyone help. and please give me a good explanation about this. life sucks right now.. were ona brink of breaking up. and i dont like it. please no foul answers. and put yourself on my shoes and her shoes.. thanks ....