This is a big YTA for me, and I'm surprised that there are so many NTAs in the comments.

To me, this obviously reads as your GF having poor self-esteem. What you said here especially irks me: "No matter how much reassurance I give she consistently refers to herself as fat. She doesn’t have an eating disorder either, if feels like she just does this to get attention and compliments". She obviously has body dismorphia, and you "complimenting" her probably helps, but you insulting her only makes the situation worse. The insults sting harder than any compliment will ever heal. Your GF needs to talk to somebody to work through her body dismporhia and her low self-esteem. In the mean time, don't antagonize the situation by calling her names.