CMJ, your girlfriend sounds exactly like my ex wife, except for the difference in age. 5 months in is a pretty short time frame to be sticking through issues like this, in my opinion.
The tough thing about speaking in terms of the "potential" in a relationship is that it isn't the current reality. The current reality is that you're in a relationship with someone who switches from nice to abusive. Sure, there's the potential that she might decide to turn things around. There's also the potential for you to win the lottery next week, but that doesn't mean you should spend your whole paycheck on lotto tickets, if you catch what I'm saying. The analogy here is that instead of having an empty wallet in the hopes of getting a cash windfall, you're forcing yourself to endure abuse and sadness in the hopes that your abuser will stop abusing you.