My Ex girlfriend slept with her ex boyfriend and I need some help Please?

ok here is the situation. Me and my girlfriend was in a wonderful relationship for about a year, we were so happy, even though she had a rocky past. Well as life goes on we were headed in different directions because i went on to college and she went to texas, a this time we are still together and really happy with the long term relationship but as time went on we eventually broke up but still loved each other cause we never stopped talking on the phone or anything. well as time passed i eventually found out that she slept with one guy but he turned out to be a dog that played her. it really shook me up pretty bad i was heart broken but i got over it some how and we still talked and told each other we love each other. ok so now she moved back closer to me and we are closer than ever but not officially back together even though we act like it and plan on having a future together, well a week went by since she returned and having a conversation somehow it ended up her telling me that she slept with her ex boyfriend whom ive had issues with before cause he couldn't let her go. This really killed me inside my chest felt heavy, she said it was a huge mistake and that she was thinking bout me telling her i love her while he was touching on her. she said that she was vulnerable. I know im a guy and i play basketball for a Division 1 college and i didnt sleep with no one but she slept with two ppl and one was her Ex! should i forgive her? cause even now i love her to death and she says shes still in love with me but idk. What should i do?