Commiserations Peachy, had that with my mum too.
She was so nasty and spiteful, and not just towards the end of her life when admittedly she DID have dementia. She also never had a good word for anyone, and the only times my siblings and I have fallen out is when she has come between us, A said this about you, F said that about you and so on.
Latterly I went to hers every day to clean, get her lunch, make her tea and do any errands. She did things like writing "Strychnine" on the sugar bowl, (she didn't take sugar and I did), she would somehow wet the bottoms of paper packages, such as sugar, flour, soap powder etc. Of course when I picked them up the contents fell out.
She used to tell my son one thing, and me the opposite, leading to my son thinking for a while I was being mean to her requests, one major one being that she asked me to take her back to Yorkshire as she wanted to finish her days there, and days before the move (we moved too) she told my son I had bullied her into moving and she didn't want to go.
Sorry, just realised I am rambling a bit, haven't had these thoughts for ages.
Just know you are not the only one, and no matter what you do it will probably never be enough for her.
Sorry for your pain.