My BF cheated on me, I kissed a guy. I need help?

Me and my boyfriend are 10 months (we're 18) we had broken up a week ago because he "wanted a break" and I said no so he said just to break up and that happened two days later I went with my friends to the beach and there was people from my school everyone was high and drunk including myself. We were in a circle and this guy (same grade) we were all talking about things and we kissed we like kind of made out, after that I only saw him about three times and we kissed again but that was it. My ex was texting me the whole time how he wanted me back and everything we had talked and he said when he left to Mexico for vacation he cheated on me and pretended he didn't have a girlfriend then in school when we argued and he kissed his old kick in the forehead. I feel so dumb now that I got back with him that's all I think about, and that one guy he hasn't been in a relationship in a while but I don't know I can't stop talking to him even though I know it's out of respect for my boyfriend what should I do? He tells me he regrets everything deeply and he really wants to make an effort to try because he wants a future but he's done to many especially before he would flirt with his "best friend" when we were together