This is very much common between best friends of opposite sex. You two are best friends for a reason. You guys, I presume, like each other’s company, care about each other, share details about what’s going on in other’s life. Emotional and physical longingness only seem natural in such case. If it was just one time thing, and you both have felt awkward about that moment of physical connect, best way to get rid of that is to talk about it. It would make the instance a light enjoyable memory, without affecting your friendship. However, if it isn’t, your friend has to make a call here about does he want you in his life more than the other girl? If yes, ideally he should break up with the other girl and be honest about all this. If he isn’t serious about this, he may still want to have ‘the new developed’ equation with you, as a ‘additional spice’ in life. Keeping the moral literature aside, you may go ahead with it in this case as well, if you don’t have any expectations from this. But this tends to be a risky path, as more often than not, the third person starts having that longingness formalized and not hidden from others.
Here’s the crux of the matter: She knows you love her. You told her so, but she already had a boyfriend, so even if she wanted to, she could not reciprocate with you. But she liked you enough to stay friends, and talk you off the metaphorical ledge. That’s a pretty good friend. You helped her out as well, by helping her see when it was time to pull the rip-cord on her failing relationship with the other guy. You also reminded her at that point, that you still have feelings for her. Probably bad timing on your part, but at least you’re sure she hasn’t forgotten that you would like more than mere friendship with her. And, in time, she has apparently moved on to yet another guy, after letting you know in no uncertain terms that, while she likes you as a friend, she doesn’t see in in the romantic way. I think it’s clear that your love for her is unrequited and nonreciprocal. Sorry, guy, but life sucks that way sometimes. But the good news is, she’s not the only girl in the world. She’s a great one, I’m sure. But she has a significant flaw: she isn’t smart enough to recognize what a great boyfriend you would be. Why would you want to waste time courting a girl who doesn’t see how awesome you are?