I cheated on my girlfriend and i want her back?

to make a long story short, she said she had to go to her moms so it was fine with me and then wen she left one of my women friends came over and we were hanigin out, then she randomly said she had like me for a while, i didnt realy care so i was just like w.e. and didnt acknolwedge it, she then began to kiss me and she began taking off my clothes, i was in complete shock and i wasnt sure wat to do, she began giving me a handjob and kissing me at the same time and then my girlfriend walked in, i wasnt sure wat to do so i pushed my friend off of me and my girlfriend walked back out. she came over yestardayso we could talk about it nd she broke up with me. i tried to explain what happened and how she came on me and i ddnt want it, but she kept asking why didnt i push her off wen she tried nd why didnt i tell her to leave!!! how can i get her back and how just what should i do about everything? help me pleaze?