I'm going to leave my girlfriend to get some sleep
Has she been keeping you from being able to fall asleep?
She won't stop talking to me.
If you need to leave for awhile to get some sleep that is cool, excess talking is really bad for feeling rested. She should be able to understand that it is disturbing not to do good by your wishes, otherwise you will not be able to function tommorow.
She's selfish.
It is understandable that you want some quiet in order to get sleep. This just sounds like something you should talk about with her when you are both well rested and probably during the day. Leaving does not sound like a bad idea at all.