Easy. Go to a spring break party. Hook up with a hot drunk chick. Get some pics of you totally motorboating the hot chick's tits and her doing shots off your body.

Send pics to your cheater gf and tell her that you're spring break partying too. (no genital pics. this could come back to haunt you)

Do Not go back to her. This is when cheater girlfriends get psycho and consider it a sexual challenge to get their boyfriends back after cheating on them. They will resort to anything to win this female challenge including trying to get knocked up or promising you anything you want