Grandmother hates going to toilet so she won't tell me when she has to go #2. What should I do?

My grandmother who I take care of in my home has been in adult diapers for years for urinary incontinence. She moved in with me last year and was going to the toilet for #2 with a cane and letting someone know to help her. It soon progressed to putting a potty chair beside the bed because she didn't want to walk to the bathroom anymore. Then she started having these "spells" every time she got on the bedside potty to poop and she would beg and plead or yell and scream and threaten (whatever she can do to get her way) to put her back in bed even though she wasn't done or cleaned up. After that she started wanting me to change her laying down instead of with her sitting/standing on the side of the bed when she is capable of doing so. I have had seveal talks with her as well as my mother about her needing to at least sit on the side of the bed to change but she doesn't want to and doesn't seem to care. We have even explained to her that if she doesn't try to cooperate and it gets too hard for me that she will have to go to a nursing home. She has dementia and thinks that we can't put her in a home if she doesn't want to go. She doesn't get out of bed at all anymore, even to eat at the table in her powerchair, which she used to do when she first moved in with me. She now goes #2 in her diaper and doesn't tell me until she is done so she doesn't have to sit on the potty. She has dementia but also has always been a manipulative selfish person. It's so stressful and it's taking a toll on me mentally and physically. What should I do?