Should I end it for good?

Me and this girl talked for 4 months. We didn't speak for a whole month after we split until we finally drunk texted each other saying how we miss each other. However, she is seeing another guy right now. We went to the same party last weekend and talked the whole time and it was great. But the thing is my friend saw her making out with a guy the night before...now all I can think of is her and this guy getting physical. Anyways, I am having a get together for my bday next week and was gonna invite her. We are both in college and going home for summer in a couple weeks so won't see each other for a while. Should I just be straight up with her and say, "I like you and I want you...but if I can't have you let me know and I will move on". I'm just tired of the mind games and want to get this show on the road. I want to be with her but I'm impatient and I don't want to wait around