Well there's this girl I've been friends with for about a year and a half and we've gotten to be really close friends. At first I felt no attraction towards her but I started to really like her. About 6 months ago we both felt some connection between us, but at the time she had a boyfriend so nothing actually happened. She told me that she liked me and I told her I liked her back. She didn't want to be fucked up to her boyfriend at the time so like I said, nothing happened. It really did tear me up inside seeing them together, in fact I think I cried once or twice. To be fair though she did lead me on a lot, texting things like "I miss you" or shit like "I can't stop thinking about you".
Been there, done that. Back off, keep some distance, don't be that close. If you're attracted, you can be somewhat distant friends, but for your own mental health you cannot be too close to her. First and foremost because it will make you hurt without the shade of a doubt; but then because it is your only slight chance to have her chase after you at least a bit (and maybe kickstart again from here, who knows).
I think you just need to decide that you want to go out with her, and behave accordingly.
Learn to seduce her, and do things with her that you do with a girlfriend.
u blew it. move on. its that simple.