firstly, if u really just a 11 years old boy, then what i can said is fo u really know what is love?? Is that ur first love? if yes then what i can said is clam down and look forward, dont let it waste ur time.
If u really want to get her back, the only way is, try to be her friend again, but dont do anyway that friend dont need to do, jsut let her know that u still care about her, i am sure she will feel that.
but dont care her far too much, otherwise she wont like it, i am sure!!!
after that , is her choice weather she will choose u again or not, just take ur time and think what u should do, but the first thing u have to do is clam down. Hope u can find a way to solve ur problem, good luck