I'm having the urge to push my girlfriend to go further sexually even after I promise I wouldn't.
What is the reason for this?
I saw her kissing another boy.
I can get the frustration of see a girlfriend cheat.    Forcing her to do sex is wrong regardless of the situation and could end up getting you in trouble.
She should have thought about that when she decided to cheat.
Your situation is really tricky. I get your mind may be all over the place with knowing that she cheated on you. But forcing her to do sexual things that she is not comfortable with isn't going to help out your situation.