I don't know if you're into country music, but right away, this Sammy Kershaw song came to mind.
It's hard to overcome the conditioning, when everybody else is telling you how ugly, fat, stupid, and crazy you are, and then the one who loves you tells you the truth, and you're like, "Oh, you're just saying that." We learn early in life not to come right out and say, if it's your birthday and someone gave you a present, "That's the ugliest thing I've ever seen." Instead, we are taught, it's good manners to say you like it, even if you don't. Thus, it gets into our heads that people who say nice things don't really mean it.
Several years ago, the topic being verbal child abuse, I saw a woman on a talk show who had been called ugly all her life, by her own mother. Let me tell you, that woman was a knockout! She was gorgeous! But now, no matter how many people tell that woman how beautiful she is, I could see it in her whole-body reaction. She just rejects it. She cannot believe her ears, because she's been told too much that she's ugly. It later came out that her mother later confessed, she knew all along her daughter was beautiful, but told her she was ugly so she wouldn't grow up to be conceited about it. If I told you everything I thought about that, this post would get too long.
My psychologist points out, "The only proper response to a compliment is, thank you." But even she says, we are programmed to think the person complimenting us is feeding us a crock.
If I were you, I don't think I'd get into a heated argument, trying to convince her. Understand why your girlfriend doesn't know she's beautiful. Don't stop the compliments, but don't get mad at her when she doesn't believe you. Tell her you wish she could see herself the way you see her, and tell her you think it's a shame that people feel so threatened by her that they have to try to knock her down, so they can feel better about themselves.