That's a tough situation, to be in.

There's a lot of focus on the outside self. Could be the culture she was raised in, where outside appearance matters more than everything else, a culture of objectification, perhaps? When dealing with such constructs, unless the person, your gf in this case, learns to love themselves, she's truly setting herself up to sabotage the relationship.

Not certain what advice to give to get her to see that. Maybe talk about fear of eventually pushing you away, because insecure behaviours will wear down a partner and breed resentment.

Maybe couples counseling?