Oh yes, she is definately NOT going to be happy if you tell her you did it again. It's not going to be ok. I'm sorry to be harsh, but I think this relationship is over for good, my friend. I would be disappointed in HER if she went back to you after this. I understand you may care for her, but your heart is obviously not completely in this relationship if you can
A) Cheat on her, and then
B) Sleep with the same chick you cheated on her with, right after she breaks up with you for CHEATING
My guess is you have some unresolved feelings for this girl you keep sleeping with, and you need to seriously deal with those. Honestly, you are very fortunate your gf came back to you after you cheated on her. Let her move on, for both your sakes.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi