How to break up

I'm going out with a girl, we are actually dating, but the girl I have always had crush started talking to me again and we are going out tomorrow.

Just thinking about her I get super horny and it has never happened with my girlfriend.

Also we have been talking all day for some time.

HOW DO I BREAK UP WITHOUT MAKING MY GF FEEL BAD? I mean, we moved to fast from "going out" to "dating" and the other girl appearing just made things worse for me.