Why can't I let go of the past?

Ok. So I was kinds sorts talking to this guy for a total of almost 5mths. We had our ups and downs and eventually cut things off the first couple of months then eventually got things started again. We nvr were official but we did the friends with benefits thing. Now he's a senior so he leaves for college this yr. anyway things ended up ending on a bad note. I still have feelings for him though even after everything that happened. Idk what to do. Why can't I just let go of him? Is there something wrong with me? Please help!