Hw do i get my ex girlfriend back after i have cheated on her and she has moved on straight away?

i didnt she was the one and i didnt know i had such strong feelings thats why i started cheating, and i didnt end it because i didnt want to hurt her, now that he has found out i cheated she ended our relationship and im devastated, and she jumped straight into another relationship with a guy who was supposedly just her friend that she says has been there for her, what gives?, she says they have alot in common and get along really well, how do i get her back and repair things, wy did she move on so quickly?, she says she feels our relationship ended 10 onths ago but she was ready then, ive told her how i feel now and she doesnt sem to care