First love I think but so frustrating?

Okay so im 18 and my ex is 16. Im going to be a sophmore in college (I skipped a grade), and my ex is still in high school. We were together for about two years and it ended about a month in when i went to school because i did stuff with another girl. We had a really great relation ship while we were together and I can honestly say she is the first girl i ever developed real, strong feelings for. Since we broke up about 9 months ago we will talk about every other month and see each other but it will lead to nothing because she still seems to be harboring a resentment over me and other girls. I try to make it work with her but it wont because shes holding a grudge so instead ive been dating and doing other stuff with a lot of other girls but I dont seem to really like any of them and I always find my self thinking about her. All we do is fight when we talk and when we dont talk she always seems to try to know what im doing indirectly by talking to my sister (who is 15) or talking to my cousin since she gos to a school 15 min away from us. She says she loves me and likes me but she wont show affection because she says shes so hurt over me sleeping with other girls. Its been 9 months! How could she be scared to show affection! I dont seem to understand how shes still hurt and I dont know what to do. If I cant make her leave me alone or show affection too me how do I get over her? Ive been with, and im not exagerating, 10 girls since her (but she doesnt know) and im not over her! Whats wrong with me and with us????