I would start believing (on faith if you don't see it) what others tell you. If someone says something to you, that is how they are perceiving you so you can't really "deny" it as it isn't your perception. Start asking others about their perception of you as "pretty" (or anything else; smart, fun, good sense of humor, etc.) and looking for what they see, in yourself. It has to be there in some way, they have nothing to gain by telling you something they don't see?

The more you look for and see good things in yourself, the more they will manifest. I'm not talking about being fake; think of some things that could improve your looks, like smiling often, and teach yourself to do that until it is a learned behavior. I've been told my eyes light up and sparkle when I do/talk about X so I practice doing/talking about X more often! That sort of thing.