I know exactly what you mean. I went through the same thing with my father. He had a series of small strokes, and with each one, he was more debilitated. By the time he died, I had had a long time to mourn losing my father.
A lot of my tears were about the life he had to endure at the end. It wasn't what he was.
But he's with God now - as your mum is. That's a better place for them, even if we feel desolate and deprived sometimes.
My mother's dementia had really increased the last few months and she did not really recognize me. I know she is in a better place and is singing in heaven now. I would have ups and downs.
I had to give away all her things and sell the house as her executor. I could not drive by the house on the drive to my daughter's after it sold.
My daughter had theorized we could just keep it and meet there for family visits! Oh my! Letting go was hard for the grandchildren as they had spent many happy hours there.
I had been with her every weekend until she fell and finally had to go to a nursing home. I had watched and suffered over the decline that they had not witnessed week after week. I now can recall the lively, interested, youngest 90 year old one could imagine instead of how she was the last 2 years..
Dealing with dementia does take a toll. Take care of yourselves.
And those are the wonderful memories of your Mum that will stay with you!