Firstly it is ok to grieve even though the person is still alive. If you need to give yourself any kind of permission for this then do.
My mom died four years ago this week.
My dad hid her dementia from us for a while. He was very clever at that, bless him, my mother and I had always been really close and I noticed very quickly. Slowly she faded. Forgot where the toilet was in her house, forgot where her bedroom was. We managed with carers to begin with, but eventually it got too much for my dad and she went into a home, where, when Covid came she passed away.
It was really in the home that the dementia ramped up.
She forgot my sister's name, had me explain over and over who the people were in the photographs in her room. But I also got stories of times in her youth that I had never heard before. Boyfriends before me dad, liaisons she had had. She was a funny old lady, but she slowly stopped being my mom.
So for a while I visited this funny lady who looked like my mom, but had stopped being the person I knew - and this was hard, but this was ok.
She called me Liam, which is not my name, and we sang childhood songs and had the same conversations over and over and even this was ok.
Some people say you grieve twice, but I think you grieve many times, and when she finally died - and I admit this to you, but not my family - I was relieved. My mom died almost eight years ago - the person who could strike fear in my heart by using my full name - the person who could knit do a crossword and watch tv at the same time - the person who could make a Victoria sandwich, a set of curtains and a matinee jacket on the same day - she had gone.
The little old lady who I was very fond of was somehow easier to let go of.
Be hugely kind to yourself OP, talk to whoever you can, both professional and not. It will be fcuking hard, but your mother is a fantastic woman, and fantastic women make other fantastic women.