Oh boy - I've been there... You've got the nail on the head when you say "I CANNOT ALTER MY CIRspamSTANCES BUT MAYBE CAN DO SOMETHING ABOUT THE DIABETES". There are two problems here: the stress the situation is causing you, and the needs of the person you're caring for. You don't say anything about the needs of the person you're caring for, but I imagine they're pretty heavy.
Number 1: Call in social services. They may be useless in themselves (other than being another shoulder to cry on), but they are the gateway to bigger and better services. Number two: if it's in any way appropriate, call in the local district nurses. In either case, be present at the interviews/assessments as the primary carer and be clear about your own needs. If home care/domestic care is offered, accept the offer. That gives you a couple of hours off. I often used the time to take the dogs for a proper walk rather than a one poop and you're back home walk - gave me a chance to clear my head a bit.
I never liked social services involvement and, since my partner's death, have distanced myself from social services. But they were there when they were needed...
Steve