Whelp: I can throw out empathy, but unfortunately, no solutions. A previous g/f ended up my fiancee, and even though she described me as "the best boyfriend ever," she still had personal issues that shattered our relationship.
I'm not trying to assume our problems are similar, but in my situation, my fiancee had confidence issues that led her to eat a lot. I never stopped complimenting her, and I gave her as much attention as I could. Looking back at past photos, I now realize how much weight she gained during that time. I guess it never mattered to me. She was still a fantastic person and was fun to be around. For those last couple months, she would come home from work always looking sad and wouldn't say much. She dragged me into arguments. I tried couple's therapy, but she didn't bite.
I knew the relationship was over when she had a "girl's night out" and came home announcing how cool it was that a random dude paid attention to her. Obviously, my own compliments weren't enough. She had a meltdown a couple weeks later and moved out in a horrific situation that would require a book to describe.
My current g/f has amazing curves and is a lot better at dealing with (temporary) weight fluctuations. I moved to NYC for her, and it's been amazing. I guess I got lucky.
If your girlfriend is eating through some issue, I hope you can find a way to help her. I wish I had an answer.
-gf