it IS infact an eating disorde and she is underweight. i AM ALSO 5"2' and struggle witht the same problem as your gf. My bf and I fight over the same thing all the time. By telling her she is ksinny over and over is not good. It may get interpreted by her as, "well if I am skinny, then wy does he have to keep telling me, is e trying to convince me of something that isnt true?" which causes her struggle to continue. You need to give her space on this one. She is obviuosly insecure about the way she looks which is where you paly a key role. Instead of telling her how skinny she is, ty saying she is hot or pretty not necessarily referring to the disorder ot her problem but still complementing her hopefully making her feel a bit more secure, at least with you. It is hard on your g/f too, Im sure. Looking in the mirror everyday may be difficult for her and this eating disorder is not easy to stop. she cannot stop just overnight. I have been at it for 3 yrs. Trying to stop for 1 yr. and cant, it is hard struggle that you need to be by her for and making her feel secure, just not necessarily targeting the problem right on...
hope I made sense and maybe helped,
much love,
lexi