How to get my Ex-Girlfriend back?

Well, This girl and I were dating for about 3 weeks, and we had alot of fun. Then, i dont know why i did this but i started hitting on her bestfriend and she started flirting back. My girlfriend of course did not know we were flirting and what not. So, one day out of nowwhere her bestfriend wrote me a note saying that when i hug her from behind she feels like the luckiest girl in the world and that I get her wet ALL the time and sometimes its just when i look at her. she said that she cant date me because of My girlfriend/her bestfriend. and that if i ever needed to tell her something just to write a note. anyway she dropped it in Sidonia's (my girlfriends) room and she found it and read it. she would nt talk to me for about a week or so. then she started talking to her bestfriend again and their bestfriends INSTANTLY.. i mean its like seriously i stopped flirting before she wrote the note and and its like WTF! anyway its been about 1 month and a half and ive talked to her about getting back to gether and now she saying she is in love with this 17 yr. old emo ******* pedaphile *****! (btw Im 14 sidonia is turning 13 and so is her bestfriend) and now they talk all the time on facebook IM and webcam with eacother and a few times ive glisped over i see him and her putting a bunch of hearts and saying i love you and all that stuff! Her parents dont care she talking to him! i really think im in love with her.... Anyway ive talked to her about a second chance and she said probably not and that she doesnt wanna ruin this relationship (with the 17 year old.) for a different one :/ and then this is wat really pisses me off, me n her were in the kitchen and she was trying to make fried pickles.. the grease caught on fire and she panic'ed and put it under water and it exploded and she got seconnd degree burnson her neck face hands chest stomach and legs.. :( and i blame myself because i knew how to pput out a grease fire and flour was sitting right next to me on the counter.. instead of pointing and saying sid, fire! i should have dealt with it myself. ive been at her bedside 24-7 literally i havent left her bedroom and ive fed her and changed her gauze wraps and stuff. and we started talking and stuff and i think that we might be getting back together.. and then she pulls out her laptop and starts talking to the 17 yr. old and he keeps putting hearts and happyfaces and stuff and she stayed up till 5 am on her laptob talking to him.i didnt help her and stay at her bedside and helping her in every way possible only because i really care about her. but that just like broke my heart that she starts talking to him. its like he is controlling her mind.. i really just dnt wat to do PLEASE HELP ME!!! i honestly think i love her..