Hi Anxious Anonymous
Only you can answer that question; it wouldn't be the same for everyone.
I have a relative who has PIP and is morbidly obese because of the medication he takes for his severe mental illness. His obesity has had a severe effect on his mobility, so he can't prepare a meal, can't walk, or wash himself, etc, so we were able to mention this along with his other conditions against some of the PIP questions.
So you need to consider what impact your obesity has coupled with your severe mental illness against the various questions.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
