I have a crush on one of my best friends. What do i do?

Im a guy BTW. Iv'e known her 4 over a year and had a huge (and obsessive) crush on her 4 a long time, she knew i liked her but never made a move or anything to show she liked me back so i assumed she didnt and gave up eventually. After that she helped me through my first (and only) break-up, helped me with family issues, etc. She's the only person I completely trust and she knows all my secrets. She isnt a very open person so I dont know all of hers but she has told me a few of them. We are both kind of nerds, but, she is extremely hot, just absolutely gorgeous... shes also funny, nice, etc. I am starting to get feelings for her again and the big problems are: she already has a bf, she didnt like me b4 so why would she now, and im really shy with this stuff. What do i do? I dont 2 ruin our friendship, but, she is the only person i fully trust and like... Im so confused