Major love problems...How can I win her back? :[ (sorry about awful grammar cant fit it all in)?

I start from the beginning.Me and this girl have known eachother for about 2 1/2 yrs, I first met her cause of music lesson in skl. far as I can remember Ive had a big crush on her, slowly but surely I got to know her .We became good freinds imo and about a year later (we were both mid 14 ), she started showing atraction; flirting a little, touching legs when we were playing etc -obvious signs. We got really close I think (still mostly only talking to eachother at music 1 a wk). As time went on we did practises alone with eachother, duets etc. Me, being quite shy (def notflirty) in the end wrote her a love letter (last xmas) I diddnt leave my name but I gave her clues it was me. However after that we went back to 1 wk lesson (we were practasing 4 xmas), and now she always with this guy, Im not sure if she loves him tho, they broke up fter bout a wk i tink, but I know there close, there always together, I see them huggin (nvr kissing) and I cn nvr her on her own. i tink he loves her