Some people are so good at putting everything you are feeling and thinking into words.And so do you, dear poster, and that's the tragedy of the situation. You set yourself up for her to knock you down. You tell her the things that are important to you, which gives her the power to deny you and hurt you.
Think of it this way - if you had an enemy you wouldn't open yourself up like that. You would protect yourself by keeping your feelings to yourself. You wouldn't deliberately show vulnerability in the hope that they might suddenly turn empathetic and be kind to you. In your situation it is your mother's dementia that is your enemy.
Whether there is an element of deliberate choice in her actions is irrelevant. It isn't fair, but no power on earth is going to change things so that you are treated fairly. The only person who can change things is you. Ask yourself, do I need this? Why do I think I can please my mother? Do I deserve better?
Believe me, speaking as the daughter of a critical and selfish mother, you can NEVER do enough for them, and they will always feel free to tell you how much you are letting them down. You cannot win with someone who is a bottomless pit of neediness. Accept that this tragedy is a result of the way she has chosen to view the world. Her expectations have always been unreasonable and are now amplified by her mental condition.
I remember the lightbulb moment when I had been metaphorically kicked once more by my mother, and was agonising out loud that there must be SOME way to make things right. My OH sighed in exasperation, he had heard it so many times before. He said:
"You've got to face it, your mother is not normal. You cannot expect her to behave like a normal person because she isn't able to. Stop allowing her to hurt you. She will always do what she's always done. YOU are the one that has to do things differently."