Best Friends Forever?

T his all started at the beginning of the school year. me and her had become friends over the summer.. and we started talking every day. Eventually, everybody knew that we liked eachother, but i didnt have the balls to ask her out. This was in November. I finally asked her out over the phone in January.. and she was the best girlfriend i have ever had. When our relationship was good, it was truly amazing. When it was bad, it killed me. Eventually, she uninvited me to her sisters party because i didnt talk to her enough. At this party, she cheated on me with a guy or two.. then got mad when i told people. She developed the reputation as a *****, and we didnt talk for a while. Then, she came to me over AIM, and told me she still wanted to be best friends like we had been before going out. I told her thats what I wanted as well, and now, three months later, school is over. She is going to private high school, while I go to public. We are the best of friends in the entire world, and we definitely love eachother. The problem is, ive told her many times that the love i have for her is more than friends, and i want to be with her again. Every time she responds with "Oh... im glad you told me.." But this girl flirts with me nonstop, and I cant really explain it in words, but honestly we are always together and flirting with eachother, yet she wont go out with me. I have a theory.. that I think she does like me, but she wants to fool around with lots of guys.. and not be stuck to just one. She tells me that we'll be best friends forever and ever, and I feel like im being used. I honestly do anything and everything for her, and if this isnt true love, i dont know what is. I feel like i should just tell her that i cant do this anymore.. but i cant.. and even if we arent going out we are good friends, but it hurts just to be with her. I dont know what to do anymore, ive cut myself and thought about suicide and my life is spiraling downwards. Somebody help.