I think the problem here is not that she's a girl, it's that it's your best friend. There's a difference. Try some space, distracting yourself, actively stopping thoughts that could be problematic etc. It might go away in time.

Yeah, that really is the hard part. Sorry, I really have nothing to offer. Is there any chance she likes you like that too?

Been there, more than once. Believe me, it's a place you don't want to be.