Need help with a girl?

I've never been the type of guy to have problems with girls. YET,for some reason i can't get over a girl who also happens to be my bestfriend. I have never cared or like a girl this much. She's told me that she wants to stay friend and I respect that. I still like her and dont know what to do. She's gotten weird around me lately, to the point where it seems as if she doesnt want to see me or say hi to me. I dont get it, like i said i've also been good with girls, but for some reason this girl ia different to me. I've had chances with other girls but because i like her i dont get with otber girls. I've even had a girl try to sex with me, i turned her down because of how i feel about her. And thats hard for a teenage boy to do. Shes more then just another girl. I truely care about her. I know I'm only sixteen but i want her to be my girlfriend. GUESS I'm a young teenager in love with a girl in which i have chance with. What should i do.