There are two ways of looking at situations like this. One, your mother's personality is being 'simplified' as dementia strips parts of it away, and there's a core part of it which is tied to you and the way she's always treated you. So you're getting treated badly, as usual, and on top of all your other concerns it's making life frustratingly difficult.
Two, your mother is losing grip on everything she's ever known or trusted. She used to be in control of everything -- for better or for worse. Now she increasingly feels like she's in control of nothing. It is the most terrifying, confusing time of her life and it's only going to get worse... until she is no longer capable of being your mother at all. Part of her may know that. If she's lucky, she won't.
One of the lifelines she's clinging to is the strongest tie any of us ever have... to our closest family. She will yank on that line hardest of all because she's drowning as the dementia tide rises and -- just as it's common for drowning folk to put their rescuer in danger -- your mother will put you in danger, mentally.
Which is why I have some happy, clappy scribble similar to this waffle hung up next to my bed as a reminder. Because god only knows (that's the notional god of Beating Your Head Against A Brick Wall) there are times when I swear my mother's just trying to put me in an early grave for the sheer fun of it!
Shes not. She's just a pared back version of the Mum who could always be hard work, who raised four children and ran a business on her own, and never, ever suffered fools gladly. Some days I probably seem pretty foolish though, repeating myself endlessly to try and get essentials done, 'forcing' her to do things she doesn't understand or think necessary. No wonder she kicks off!
On a good day for me I'm only dealing with a muddled old lady with dementia. But that's a bad day for Mum, because she's under the water somewhere. On a bad day for me, Mum's at the surface, thrashing around and aware enough to resent the way our roles have reversed. A lifetime of her being the matriarch is gone. She's become the child, and boy, in her situation I'd resent it too!
Maybe none of this is applicable to you and your mother. Maybe I'm just projecting elements of my life onto you. But one thing is certain... one day you'll miss this exaggerated but still identifiable version on your mother. Breath. Count to ten. Try to remember something nice she did for you when you were young. And accept her anger and manipulation for what it is... a cry for help that nobody on this entire planet can give. We can only give time and patience... and cling to lifelines like this forum when it all starts to wear us down.
Good luck. Caring over a distance is very tough, even if you have carers to act for you.