BIG relationship problems (cheating) please help :(?

Why don't you just contact the girl you supposedly kissed and have her explain to your girlfriend that you didn't kiss her? She would be the best and most believable person since you are accused of kissing her. Also, since her boyfriend was not far away at the time, he would've probably noticed you kissing his girlfriend and gotten angry with you. Have the girl and her boyfriend vouch for you and there is no reason why your girlfriend should STILL not believe you. If she still believes that you kissed the other girl, then she isn't worth your time.