Most Helpful Girl
Sorry that happened to you. There is always a reason. She is just not telling you right now. Give it time. She may tell you one day.
For now though, eat ice cream, watch funny movies, listen to good music, hang out with positive friends and family and try not to talk about what happened. This should help relieve the pain or at least take your mind off of it.
Most Helpful Guy
You're 15, romances this early RARELY works out in the long run. You may have outliers that challenge that but it happens. What I would do in your position? Talk to other people, talk to other girls. You'll realise that there are better people out there than your former girlfriend.