tottaly ignore her dont give her any attention at all... if she misses you then she will contact you, dont take her back straight away if she does make her earn it from putting you through this sh1t and by no means expect it. From what you have written and the way you have written it, it sounds like you got way to intense way to quickly, which pushes her away from you.
I dont know the girl so i cant say if she wants to break up with you or is cheating and to be frank neither is certain although as tinkerbell99 said it is usualy the case that it is one of these two things.
if you want to gain a better understanding of how relationships work try reading "The Game" by Neil Strauss its about a pick up artist but in his writting you gain an understanding of what women want and need in order to want to stay with you.
best of luck mate hope it turns out for the best.
DM