Crazy love triangle..?

A few days back I realized that one of my female friends, who I know for a fact has always and still does have a thing for me, would make the perfect girlfriend and I was starting to develop feelings for her over some time now. OK now here's the thing I asked her out and she said she was flattered but was in a relationship. I totally had no idea that she had a bf so I did some digging around with some of my boys that talk to her and turns out no body but one knew and that one said he accidentally heard about it while she was tell her bff in class but other than that no one knew and he said this was like a few months back and since I couldn't make any sense of it I flirted the answer out of one of her friends that she's not so close to anymore. And I know its the same guy but now the thing is she would always flirt with me and try to hang out with me but why would she keep it a secret? What do I do in this situation? I will chose best answer with an hour and a half.