In my experience, a girl will never admit she is pretty unless she is vain, or unless she is comparing herself to another girl (aka, "oh I'm prettier than her!")
There isn't really anything you can do or say...it's up to her to make her decision about herself, it's free will. All I can say, is by showing her and keeping on saying it it will make her feel special and pretty even if she won't admit it.
What I mean by showing her is if she buys a new top or she comes over and she looks gorgeous say why! My ex used to go "omg! You look amazing! Look at the size of your waist! It's tiny!" and would then grab my waist and kiss it. I'm no slim chicken, but it made me feel sexy when he did it - it was more than words, he was so attracted to it he was willing to touch and kiss it. Does that make sense...?
I think it could help if she tells you specific reasons for why she thinks she's not beautiful. Every girl is different with things like this. Some just say it so they can fish for compliments, and some really do have low self-esteem. I'm one of the latter. I really wish I could come up with more to help you on this, but I'd have to know your girlfriend better.
How long have you two been together, and how old are you? This info could possibly help everyone here a little bit. :/
You tell her to SHUT THE HELL UP! Because she is beautiful(:
you keep telling her she's beautiful! & if you love her you'll support her into trying to change her viewpoint! All women should feel beautiful! Just because you're not supermodel skinny don't mean you're fat!
Dude if this still don't work I suggest a therapist. She needs to learn to love herself & her body as much as you do. This is CRUCIAL! If you don't love yourself how can you love anyone else? she should either do something about it or accept it. But if she chooses to do something about it just send me an e-mail & I'll tell you all about nutrition & wellness(: it really works.
She seriously needs to stop being depressing though. For Real!
Seriously, wtf, you went out of your comfort level and she cut it down?
Tell her again, but if she denies it say this, "Hey, (your gf's name), I like to think that when I'm serious about something, that people will respect what I say. Do you think I'm just kidding around when I say you're beautiful? That I'm trying to mess with your head? *pause for a moment* Or do you think I say it because I genuinely believe it's true?"
Put emphasis on that last sentence.
Nothing. If she doesn't feel it, then it don't matter what you say