How do I get my girlfriend to believe she's beautiful?

I agree with the other girl!
well well ok where do I start
sit in front of her, look into her eyes, and play with her hand or hair or something! Whatever she likes the best just be gentle do it slow :P then tell her why you think she's the most beautiful girl in the world for "you" hehehe ;) then like when ur like talking about it
ok ok so for example talk about her cheeks and when ur talking about it touch, it feel it, let her know that you mean what you're saying to her! Don't make jokes don't let her speak if she tries to just put ur finger on her lips and go like "shhh. " yeah you know xP
no jokes are allowed it just brings a funny mood into it and if it becomes funny then it won't be AS romantic and and and be slow! And if anything funny happens just SMILE and go back to what you were saying don't talk about the funny thing
practice what ur gonna say first to make it more perfect :)

You need to express it in other ways besides just saying "you're pretty" or "you're beautiful". What is it about her that makes her so beautiful? You need to pick out her unique aspects that make you so attracted to her and tell her. That way it's more personal and believable.