How do I get my parents to accept a larger caregiver pool?

I am a 65 year old, only child, whose parents live about 200 yards from my rural home. They are 89 years old and unable to do most household things, including cook, clean, laundry, drive, shop, and manage their financial affairs. Dad has dementia and macular degeneration, and mom is frail - uses a walker and hardly moves from her chair. Their granddaughter is assisting them 3 mornings a week for 3 hours until she finds a new job. Other than that I am "on-call" 24 x 7. I am tired..............I haven't been away from the homestead for longer than 8 hours at a time for 2 years, and then, of course, I have my cell phone.

I am getting very resentful, and depressed. My health has deteriorated, and I can never relax. I am tired................, feel guilty for being resentful and tired, and am beginning to dread having to go over there most of the time.

When I suggested that we increase our "circle" of caregivers to relieve some of the stress there was a horrific argument and I told I was pretty much a "disappointment", and just wanted to "get them out of their house", and they were "fine" and there was obviously "something wrong" with me that I wouldn't leave them alone, etc, etc. I can't have another confrontation like that one, ever.

So, does anyone have any suggestions for me? I could sure use some ideas before I lose the end of the frayed rope I am hanging on to.