I beat up my girlfriend because she cheated on me and lied about it
Are you sure she cheated? Did you ask her about it?
It's so obvious, I found her naked in bed with someone else
While that is a horrible thing to walk in on, that still does not give you permission to hit a girl. I understand how upset you must have been, but you can get into serious trouble for hitting a girl in any situation if your not being threatened.
She's a slut, so she deserves it.  I'm going to jail because of her. Her dad is going to kill me when I get out..
This sounds like a bad situation, but your violence made it worse for you.   You need to focus on not being violent in the future and doing what you can to move forward.