Most Helpful Girl
Congratulations you are an idiot and an asshole! Guess what there is nothing you can do because you fucked up! You have to tell your gf because she will find out one way or another and she will want to know why you didn't tell the other female you had a girlfriend and why the hell you let her in your house when she had been flirting with you!
Oh by the way you deserve whatever you get because you can tell a girl no! Fucking genius!
Most Helpful Guy
Just be honest bro, tell the girl you kissed that you have a commitment and apologize to her then tell the prom girl "look we need to talk" take her to a private location and tell her how you feel about her then tell "but there's something I have to say" and tell her how much you regret it and that you care for her and then come out with the truth, she won't take it easy but at least you have respect and dignity and if she forgives you well maybe everything will work out, chick's dig honesty!