What do you tell a girl who thinks she's ugly?

Don't. If you girlfriend is hot or ugly it shouldn't matter as long as she's a good person. Breaking up with someone because she doesn't look good makes you a jerk. But if you really don't care and you want to break up with her for the way she looks, at least recover a piece of whatever is left of your heart and don't tell her that's why your breaking up. Say something like, "Look, I'm sorry, but it's just not working for me." or "It's not you, it's me." Even if you're lying, it's for a good reason. You shouldn't care what your girlfriend looks like as long as she's sweet, caring and loves you for who you are. Not what you look like.