Middle school relationships. PLEASE HELP!!!?

so at the end of 7th grade, i started to like a girl named mel and i didnt know that at that time she liked me back. so we were flirting the whole summer and when i came back to my town (cuz i was at my grandparents house for like a month) we hung out at my house and kissed eachother a couple of times. I was gonna make out with her but when i leaned in her cell phone went off and she had to go. i asked her if we were going out and she said that she didnt want a boyfriend in the summer. okay so i left it at that. when school starts i ask her out and she says that now she wants to be friends only even though she was crazy about me the day before i asked her out. The main reason why she said no was b/c she liked somebody else and i felt like sht afterwards. shes really really nice and funny and cute overall and i think im in love. so anyways there going out for about 2 weeks now and yesterday at a party i was at. she h/u with him right in front of like 30 people on the couch for a straight 5 minutes which disguisted me and i felt heartbroken. now shes going to aruba for 2 weeks and i miss her like crazy yet i dont know what to do. How can i get her back. her present bf treats her like shi.t and yet she always cries to me on how much she wants a boyfriend that cares about her when i could be that person. i love her so much and i want her back. i cant stop thinking about her. WHAT DO I DO?