You're hardly so fat you're about to die from it. In fact, you're probably not anymore overweight than I am, and I have absolutely no weight-related health issues, so blaming everything on your weight is just fat-prejudiced crap.
I think therapy in your time of grief would help you. I don't know that it will help you deal with your constant pain and D all the time, but it may help you feel less hopeless and depressed. That goes a long ways towards making an illness a tiny bit manageable.