When your elderly, demented mother, who has 24/7 care at home, places demands on you that you physically can't meet, how can you be patient?

I am on disability & my husband is also ill. I have 2 siblings who don't help as much. My mother makes me feel guilty after I have been helping her most of the day & I attempt to go home to rest or make dinner for my husband. She will say let him wait. She says things of this nature even though she has care at home. She tells me not to call any friends. She will even tell me not to go to physical therapy that is much needed. If I attempt to make plans with my husband to go to church, she will try to sabotage the day.

I visit her least two to three times a day and call her every two hours when I am not with her. If I don't call her by a certain time, she becomes mad at me. She criticizes me by saying I should get a job, even though I am an attorney who was hurt in WTC Bombing and had to go on disability due to 2 spinal fusions in my neck. I am now getting increased pain in my neck. She will say very hurtful things to provoke my anger. When I do get angry, she always smiles. But then I become very guilty. No matter how much I do I always feel guilty. I always feel badly. I know that I'm doing more than enough, but I always feel badly. I never feel good about what I am doing for her. Of course, when I do lose it, I feel badly about that. Is there a book about patience when the elderly parent/patient becomes too demanding?