Update:
I'm almost certain now that she and I won't happen. She likes her boyfriend's brother (D) a lot, I can tell by the way they talk. And D sort of hinted at it that he likes her but wouldn't full out say it, and then he admitted it to me. I was telling him that he should just tell her, because he would definitely get a yes. I think it's really going to happen between them, and granted at least they'd be good for each other - seeing the way they get along.
On the other hand, I'm pretty upset about it. There hasn't been a day I haven't thought about her for over half a year, and I've been working up to it, building up to it slowly, patiently waiting before I even hint that I like her that way, and then... nothing.
What I worry is that I'll never feel for another person what I've felt for her. She was everything I've ever wanted; beautiful, many common interests, a friendly/loving/caring personality, every time I have conversations with her they just flow so smoothly and they're fun moreso than any person (male or female) I've ever met. -- The worst part of it is that she used to like me, I'm 99% sure I wasn't reading into things, there were so many times she hinted at it and looking back it was crystal clear, I was just in denial when she'd give these signs (and didn't act on them because she's in a relationship).
On the bright side, the whole thing was a learning experience, though. I learned to not be afraid to be a little flirty, I learned to not be so reserved about myself, and I learned to accept the type of person I am (ultimately defeating my self-doubt and social anxiety). I think it's good preparation for next time I run into a person I get feelings for (hope it happens again).
I don't know what to do now. I feel like I've finally gotten the tasty treat at the end of the Rat Maze and it turns out to be plastic. Should I just wait to fall in love again or should I just start talking to strangers and see if anything goes?
....
PS I'm still going to try to ask her out for coffee when the next opportunity arises, I don't think I have a chance but I just want to. I'd at least feel somewhat fulfilled.