I have to be in the same cl as my ex-bf and he flirts with other girls?

we went out 4 3months and we just broke up his in my hoom room for are freshmen year we dated i middle school (last year 8thgrade) and i still kind of have feelings for him i really need to get over it i know..... hes moved on why cant i (in my heart)im dating but still cant keep my ind off of himI*