I have a similiar situation. I got a female best friend like that, I knew her ever since high school, when she moved from another town. I had a crush on her, but she brushed it off. We stayed in touch over the years, she even stayed at my moms crib while I was away in the Army. She got mad close to my family, she even considers my moms to be her mother. She's the only female that has a matching tattoo with me. Years later, she came out to visit me, one thing lead to another and we wound up having sex. At first I was thinking maybe it would work out, but she had second thoughts about it, so we let it ride. Fast forward years later and we're at a party, both drunk, and next thing I know, we going at it again. The morning after was weird, we talked about it and she felt that although she loved me and considered me to be one of the greatest guys ever, she valued our friendship more than anything and she didn't wanna risk it entering into a relationship. I understood her position (though I didn't necessarily agree) and I backed off. We're still friends strangely and to the outside looking in, someone would be like "Damn, ya'll SHOULD be together". I don't know, maybe it's just one of those things where if circimstances were different, it would be me and her. But I'm just saying, you not the only one.