I have the urge to cheat on my girlfriend.
Why would you want to cheat on your girlfriend?
I know she has been cheating on me.
Just because someone cheated on you does not give you the right to do the same back. You should just break up with them and move on with your life.
It will be last time, I promise.
You shouldn't do it at all. Just because she's in the wrong doesn't give you a pass to do something awful to her. You should confront her about this knowledge and see if you could work past that.