Why is my girlfriend keep nagging and being annoying just because i cheated on her?

She is really starting to annoy me now. She keeps pushing me and I retaliate, because she is always complaing and shouting in my ear. So what! I slept with someone else, Is that a big deal. She acts like a immature child sometimes and It annoys me to hell. She needs to grow up and not act like a brat. She is fine when it comes to borrowing money from me, but when I do something she gets on my case and that is what makes me angry. She wont leave because she is too weak and she hasn't got the guts, but that's what you get when you grow up in a one parent family i guess.