Please help me, i dont know what to do?

Okay iv liked my best friend for a while now, let's call her A. First it started off as a crush, I would hardly speak to her cause I was too nervous, but I eventually manned up, and had a laugh. Our relationship between us was so much better, playful flirting, but I ruined it for myself, as I stupidly did not ask her out, I wanted her too, just in case I did and she said no. So yer, I lost my chance, and we both went off to separate unis. We remained friends, but I started to like another girl, let's call her B. I now know that I only moved onto B as away of getting over A. I realised she was just not my type, there was nothing there, I wanted her as to fill the void u would say. So I got over B, but one day, A tells me that she misses me, so arranged to meet up which we did, but it reminded me of the old times and I was dragged back into the cycle. I started flirting with her, and just last week out clubbing we had a great time together. So I went to a party last night, and me and her were having a laugh, but then she told me she was seeing someone, I don't think it serious, but it just really hurt me. I feel like I'm fixated on this girl, that I can never get over her, she's like a curse too me, I keep falling for her. I'm in love with her, and I want her so much. So iv reached a crossroads, carry on with her, wait out and see what happens with this guy, or start fresh. I really dunno what to do, I keep asking myself this. What would you do if you were in my position. And sorry if this was quite along read, I just thought you should know the story before you offer me some much needed advice. Thanks