A promiscuous girl wants to be my girlfriend. I like her too. How do I help her learn Monogamy?

She is too young for you. Even if she could (not likely) become faithful to you, it would not last. She will want to finish sowing her wild oats in a few years and by that time you could be stuck paying child support for the next 18 years. She is not your soul mate, she is just a young girl that you have a crush on.