I agree with the idea about seeing a counselor. Only the very wise and committed can resolve these kinds of things on a chat board, or with no outside help. There may be other factors at work you aren't very aware of, that a professional may be able to help you with.
Please be aware, this person loved you when *you* didn't look so special, and deserves the same courtesy. I can understand that if a smell is nauseating, it is nauseating, but I don't buy the idea that what attracts one visually is carved in stone. What attracts one visually is a learned response and culturally conditioned. ( I just went for a walk outside and three, count them, THREE separate cars full of Hispanic men rode by and made approving comments about my figure!!) Since I have been thinking critically about the weight issue, I have changed the way I see fat people, and not everything looks so hideously ugly to me anymore. I like the way the models in magazines like Mode and Grace look. So you *can* change the way you see.
My first bf looked like a cross between Santa Claus and a walrus. I'm not kidding. I really did consider him ugly. But it is the whole person I respond to, and as long as he treated me the way I wanted and deserved to be treated, I was fine with that. And talk about a person who did not take care of himself!! OMG! Here was a person with diabetes, depression, and high blood pressure, who never even tried to exercise. And I won't go into what this person ordered at restaurants ... but I have been there myself and therefore I restrained myself from lecturing, because anyone who is fat can tell you, negative judgements from others make things worse, not better. If all you are able to feel good about is looking thinner, whether you make ANY other physical gains or progress or not, that is very discouraging and a big reason I gave up on myself and gained so much weight several years ago. Now that I can focus on STRENGTH and not SIZE, I can feel happy and motivated no matter how I look. (And that's a good thing, because I will never look like the pics I see in here!)
I do have one question. As a member of NAAFA, I talk to plenty of people who are doing what by all manner of common sense "OUGHT" to work, according to the popular wisdom (none of them have visited here), but it hasn't worked. They're still fat. And so am I. If by some quirk or decree of God, your girlfriend were to make diet and exercise changes and still be forever unable to change her body shape, how would you feel about her then?
How would you feel if someone you were with left you because of the way you looked?
I am not trying to sound judgemental here. I am just trying to get you to try to approach the issue in new ways. I have found some good-looking people (my own grandfather, for instance) to be some of the most rigid and judgemental, and some of the most lacking in how they treat other people.
I guess what I'm trying to say is, if you're going to leave this person, make sure it's for the right reasons. For me, personally, having been through what I have in my life as a woman of size, the right reason is *never* going to be about someone's outward physical appearance, and I require in my personal relationships that I be treated the same way.
Good luck. Let us know what's happenin'.