My girlfriend has gotten fat....?

I have been dating my girlfriend for a little over two years now and she has put on a considerable amount of weight. When we met she was around a size 2 and now she is around a size 18. When we met, one thing we had in common was that we enjoyed exercising. I take care of myself; I eat healthy, I go to the gym at least 6 days a week, and I generally stay within a healthy weight range. I have encouraged her as much as I can, I have bought her a membership to my gym, I cook her healthy meals, and I encourage her even if she only loses one pound. When I do cook her healthy meals, she will pick at it, then say she is full, then ten minutes later start "grazing" for anything sweet we have around. She constantly snacks on foods that are unhealthy and will actually get up in the middle of the night to eat ice cream, just so I don't see her. I don't judge her eating habits, but I do let her know that it disappoints me when she eats a whole bag of candy. She thinks just because something says "low calorie" it is alright to eat. I agree, but in moderation. Of course she is self conscious about it and constantly calls herself fat. I tell her, baby you look beautiful, but now I find myself disgusted to even look at her. The whole situation is making me depressed. I constantly try to look past it, but it has been harder and harder everyday. She tells me that when she dated here last boyfriend that they broke up because she put on serious weight. She lost all the weight after the breakup, but now I feel the same way as he did. I feel a bit resentful about it. That is causing me to feel depressed and be very distant. I am a very loyal partner and stay by my partner through thick and thin, but I don't know how much I can handle. She expects me to act a certain way and expects certain actions out of me, but when I bring up any talk of going to the gym together, or go running, or go for a walk, it becomes of fight of "oh, so you're saying I am fat". Needless to say, our ual relationship as fallen completely off the map, which drives me insane. I don't know how to handle this situation at all. I love her with all my heart, but it makes me irate to think that I would be so shallow as to leave her because of the way she looks.