call her out on it. "why do you keep saying it
"she might be sincerely looking for reassurance that you like her, or it might be self-sabotaging: "I keep telling him so eventually he will realize I am an ugly fattie and will break up with me"
for a while, my GF used to point me at any hot girl on the street. the first few times I played along, acting as if those girls were the hottest shit in town. after a few more tries, I reassured her that I liked her and thought she was hot. then in the end when neither thing stopped the behavior, I called her out on it "this is sick. why do you do that?" and she said it's a self-sabotage where she subconsciously wants me to find other girls hotter than her, so it will fulfill her prophecy of not being worthy enough. then I asked her what I should do as a reaction. she wanted me to call her out on it, like I had done. after that day, she stopped.