In love with taken girl

Hi, thanks for reading my post first off :)

Well to start off i get massive mixed emotions about my situation, it all started with me and some other guy that where trying to go out with her, then she came to me for advice because she wasn't interested in him. So i played neutral in the whole thing and gave some advice on what to say to him.

Then i missed a couple of days of school (because of cold) and when i came back she told me about how annoyed she was of him and how he said that "Ok, ill get a girlfriend by the end of the day" this pissed her off and she said the something just about a boyfriend. Now i understand that right then i should of stepped up and asked but i wasent sure if she was really going to listen to him.

Much later i see her with some other guy, i havent even seen or met before (not that that matters) and i wasn't sure what to think at that moment, only that i waited too long. So at that point feeling stopped there...Until concert day, i bought 2 tickets for a concert when she was single. The whole time she acted as if we where going out or something, i was holding her the whole time and we had so much fun, also when we where talking about this couple that didn't match at all she tells me "you know, we match"

That really caught me off guard and i was very happy but confused because that could have ment anything and i might be interpreting it wrong but still all the signs where there. And now im back to liking her again and am not sure what to do, we act flirty all of the time but she is still going out with guy, so im sure what i should do (if anything)

If you read all of this i really appreciate it, seriously thanks