Welcome to Fluther.
You’ve gotten some good advice above, if you’ll take it.
You seem to be falling into a mode of behavior that a lot of men and boys get criticized for – and rightly so! That is, the scenario where a girl smiles at a boy, he falls in love with her over that, and suddenly believes that she’s “his girlfriend”. Or a girl pays a compliment to a boy, and he then believes that she’s in love with him.
That can certainly go the other way, of course, but girls have generally seemed to me to be more level-headed about this sort of thing. They can do everything that boys do in this regard… but they don’t seem to do it as often. Well, it looks like you’re doing it.
Now, if you have a best friend who’s a girl, then of course you do have a relationship with her. Unfortunately for you, it’s not quite the relationship you want, because your feelings are different from hers, and you want a more romantic relationship which she has made clear she does not want. In boy-girl terms, you’ve been friend-zoned. It’s up to you to either accept that and be “just friends”, or, if it hurts you too much to be so-close-and-yet-so-far, then for you to put distance between yourselves. (Whether you tell her of your feelings so that she’s not hurt and mystified to lose her friend if that happens, well, that’s up to you.)
It’s sad sometimes when we find that we can’t have just what we want, but it’s part of growing up, too.