That is a really tough situation, and I'm sorry you are going through it.
I guess for me it would in some part depend on what the reasoning behind breaking up with you over the issue of your weight was. If it was (honestly) because of feeling concerned for your health / well-being, that's one thing, even if the method of expressing said concern seems misplaced to me. If it was purely because of looks / physical attraction, it's a lot harder... it is of course true that physical attraction plays a role in a close relationship, so if she's struggling with that it would certainly make the relationship difficult. But like the others said, is staying with her really the best option if that's the case?
I certainly don't have my own (rather different) relationship issues settled to my liking by any means, but I do have some recent experience with "worrying myself sick" over them. This fall I reached a breaking point with my (for all intents and purposes) boyfriend, and told him that I needed a break. The following day, my health went to ****. Presumably as a result of stress / relief of stress from finally taking action about the relationship, but I broke out with eczema / hives / rashes all over my body that led to uncontrollable scratching even while sleeping (when I could sleep, which I was managing for about an hour at a time!) And then that led to skin infections, which led to me taking antibiotics that whacked up 3 other systems...... It settled down eventually, after about 3 weeks of ****. So.
Whatever steps you need to take to keep yourself from stressing yourself sick about it, do. I'm sure you must love / care deeply about your partner, but you need to think about yourself too. Keeping yourself as healthy as possible, mentally, will probably be better for both of you in the long run.
