this is a problem for A LOT of females and unfortunately for some, it's a life sentence.
this is not your fault by any means. poor body image plagues most women. many believe that losing weight= looking great, but beauty exudes from inside. she's lacking a clear perception of what reality is.
perhaps try taking her to a spa treatment for some pampering.
DO NOT overload her with compliments, she'll just keep wanting more from you..but what she needs is self-confidence and learn to love herself.
"We all take different paths in life, but no matter where we go, we take a little bit of each other everywhere."