I need help showin my gurl dat i am a changed person and will neva cheat on her!!! can someone help me!!!?

I just started goin with dis gurl bout 2weeks ago. I Really like her and i dont wanna lose her to something so stupid. There is a rumor going around about me going with another gurl and i also have a history of messing with alot of gurls. I'm trying to change my ways and be with her. So can someone tell how i can show her dat i am a different person now.