What Should I do with my girlfriend?

We have been together for roughly a year and a half. We started talking on Valentines Day of last year and are relationship has blossomed from there. I love her more than I can say and one day I believe I would like to marry her and she tells me she wants the same. I want her to finish college before we make that kind of commitment though. I truly believe she loves me and I love everything (for the most part) about her, nobody has given me this feeling ever. This past weekend she was at a party I couldn't attend, I really didn't know the people and I had plans with other friends. Well two nights later she told me that she made out with one of her ex-boyfriends and somebody that she told me was more like a brother. I was highly confused and upset and she couldn't stop apologizing. I believed her and told her that everything was ok even though I'm having a hard time trusting her knowing the fact that she will be at another party that he will be attending for the Fourth of July. I don't want to be the type of boyfriend saying "hey I don't want you to go" but at the same time I really don't want her to go to this said party especially knowing the fact that alcohol is going to be involved. She promised me that nothing was gonna happen but also told me that she still had some feeling for this said guy. After thinking for a few moments she then told me that she thinks she wants a break and for us to be just friends because she doesn't want to hurt me. I don't understand what I should do, I love her with everything I have but I don't believe breaks truly work and I don't foresee us returning from this one. I don't want to let her go cause I do believe she's the one but I don't want her to pull away and break my heart. Help me I really don't understand what I should do.