My ex has told me to stay out of her life completely...?

I loved this girl so much...she cheated on me..and got back with her first love..I did not ever do anything wrong for her to just drop me on my head matter of fact I even tried to understand her feelings and I kind of let her walk on me..then she would still talk to me when she was with him..tell me she loved me blah blah and all of the sudden she would get cold and I could not tell she even cared..then one day she told me she has chosen him and I need to stay out of her life..I just do not understand why it is so important for her to just be rid of me completely like that..Just wondering why it has got to be that way.