She's flirting with me and I have a girlfriend, can't stand it...?

I won't preach to you what I think because I think cheating on someone that you're with is wrong, so I won't. I just want to say that it is your life and whatever choice you make you have to think of the consequences. Just because this girl is y and flirts doesn't mean you have to like her, I would tell her to back off. But if you prefer her over your girlfriend then go for it, it's your decision and only yours just remember to think it through thoroughly BEFORE you do anything you'll regret. Good luck! :)