How do you cope with seeing your parent slowly lose their mind?

My mother is 94. My brother and I and paid caregivers have been taking care of her for the last two and a half years and now she is in assisted living. Her mind has slowly been going but since the move seems to be plummeting downhill. This morning, she didn't recognize my brother at first. She is frequently crying when visitors come in, and seems panicky but slowly comes around. She frequently says things that don't make any sense whatsoever and obsesses on a new subject every visit, almost always having to do with just her.
This is my mother--we used to eat lunch together every day, talk about current events, work on the crossword puzzle together, keep each other up on the family. I know this happens. I know people get dementia but it is like a nightmare that doesn't end. I can't help but think about what used to be, and I know that's not helpful. How have people managed to survive this slow, painful loss of their loved one?