How do i get this girl to like me?

I really like her, I have been supportive to her when she has been sad and I am sort of naturally nice to all girls and when she feels ugly or fat or whatever is getting her down i tell her how beautiful she is because I genuinely care about her feelings and she sais i'm the nicest guy she has ever met and that i'm her best friend. Now i'm not sure if this is flirting or not but we have been teasing eachother about little things (who's taller and more silly etc..). And sometimes saying silly suggestive things. I actually thought she might have been attracted to me, and what was better it would have been for my personality, I'm not really that much of a looker and i'm not terribly muscly either (she told me she loves muscles) and i carry a bit more extra weight than i should (but i have lost 4 kilos recently, and still going) my hopes were shot after i asked her to tell me who she liked she told me it was this guy she works with who is a year older than her, I've met him, he is muscly and attractive. I'm not sure what to do now, she is also my best friend and I'd hate to loose that just because i want a relationship with her. This is really doing my head in, i have tried tricking myself to belive in other girls and just realised that i don't know them as well as i know her and that somewhere along the way she really got under my skin. Worst of all i'm 15 and go to school with her everyday and we are in alot of the same cles and it's just heaps awkward because i htink she knows i like her. This really sucks for me. I'd like any suggestions or help or advice you can offer.