Girlfriend mood swings?!?

Ok so i screwed up with my girlfriend big time, i got another girls number while we were going out and then told my girlfriend i needed a break so i could be single, i hooked up with the other girl while we were on a break. After all this happened over 3 weeks my girlfriend took me back, we have been going back out for a week now (over a year total for our whole relationship) but heres the deal...sometimes shes totally in love with me like, i love you babe, i miss you, i wanna be with you, lets be together forever, and then other times she just thinks back on how i hooked up with another girl and is like, dont talk to me, i dont know if i want to be with you, your an idiot, stuff like that...so i dont know what to do about it, because sometimes our relationship is great and sometimes she thinks back on the past and then shes totally on the brink of breaking up with me, what should i do about this whole situation?