I want to break up with my girlfriend but I don't know how.
What makes you want to break up with her?
I feel like I'm her slave.
Have you talked to her about how you feel? Try to talk through this issue with her before deciding to break up. She might not even realize that you feel this way.
I don't want to waste her time.
It is not a waste of her time if it is this important to your relationship.  It would be much worse if you broke up with her, or if you continued to be miserable, so as long as you talk to her calmly, she should understand.  If she doesn't treat you any better, then you can leave her without worry.