My girlfriend cheated i need help?

well we were talking and doing things for about 2 months already.. talking about how much we like one another...how we've never been so comfortable with someone..sex..all the good things. it was all so perfect.there was defiantly something between us and i fell in love with that. so we became official. so 2 months later i find out during that time were i fell in love with her she had sex with my very close friend almost brother (he didnt know we had anything going on, all blame is on her). so now im so hurt and confused because what i fell in love with is all false. and i think the only reason im still with her is because im still under the impression of this non existent thing i fell in love with. she says shes sorry and broke down crying and stuff when i told her we cant work because i can never see her the same again so she obviously regrets it. but regret doesn't erase what she did. she said she was 100 faithful when we started to say i love you to one another but i would have never told her i loved her if i knew this ...idk what to do. i really thought she was the one i never felt this way about anybody ...but what started it all is fake...idk man ..this is balls.