How to handle a Bratty Girlfriend!?

It's clear to me that you're not with someone that makes you happy. Though I try to handle situations where people don't get hurt, you truly need to set something in this girl to not make her so "bratty" as you say. Try to talk things out, if possible, but if that option doesn't work out, and if she gets angry at you for being yourself, then you need to break up.