Should I back out of this? What are my options for Mom's care?

I have been taking care of my 91 y.o mother with 'dementia' [found her a great assisted living, paying bills, driving 1 1/2 hrs all the time whenever she wants something] We have always had a communication problem and now that she does not remember anything it is becoming a trust issue, like I am not sure if its the dementia or a lie. She has always lied, bending the truth, and I am am into reality!
Now throw in the 39 y/o grandson with a drug problem, who has past legal issues. Every time he visits her, there is a visit to the bank, something missing. She does not tell me the truth. I went yesterday and her car is gone, and he took it. He took her to DMV etc. with his girlfriends name on the pink slip. She also had her diamond wedding ring missing when he visited, she accused another family member of stealing it, and we found out he gave it to his girlfriend. Said she did not remember.
I am sick of it and want out, but I am afraid he will take everything from her. She blames me for 'not letting her do what she wants' not giving her $ [which I always do] She does not have much and that is why I am doing this. He takes he to the bank, gets copies of her accounts and tells her I have stolen $. I am fed up! I do everything and I get accused of horrible things-I don't deserve this.