How can i heal my girl heart?!

First of all, why did you cheat on her? Were you drunk or was it just a mistake? Let her calm down a bit, and then go talk to her and try to explain. But I think that she didn't mean when she said she doesn't love you the same way as before. Love doesn't just disappear like that. Borrowing a quote from the Bible; "...faith, hope, love, but the greatest one is love." (If you're not a Christian but an atheist, then sorry for borrowing that).

Of course if you cheated on her on purpose, your relationship is probably over, especially at your age. She will heal, find a new boyfriend and continue on her life.