We need advise about mom

We have been moms in home caregivers for the last five years. Her health has deteriorated badly, as she self neglects herself. She hasn't showered in over two and a half years. I wash her arms up to her elbows and her legs up to her knees once a month before her doctor comes for his monthly visit. Her torso and hair hasn't been washed and her hair smells like vomit. She has dementia and Alzheimer's, along with a long list of other illnesses. She refuses any type or form of help, because everyone and everybody aggravates her. We are so overwhelmed and burnt out, we don't know what to do. We know she's neglecting herself and tried talking to her calmly, but nothing works. All she does is complains and everything that comes out of her mouth is negativity. She doesn't want to help herself, or anyone to help her. She treats us both like we are dumb and stupid, (and yes we know she can't help how she acts, but it still gets to the both of us) Can anyone give us some type of advise please?