What should I do about this guy ?

So I liked this guy && he's a year older than me. We went out && all that for a while but broke it off cuz I was leavin over the summer && you know those summer flings. But that was last year. When I got backk everything was as before we started going out but then we started to fight a lot. Then he started going out with one of my good friends and we would all go out together with a group of ppl and she would make us talk to we're friends again. But I don't know why but I don't like the fact that he's going out with one of my best friends. I don't know what to do anymoreee! HELP !