I don't have a lot of firsthand experience with grief--but your statement tonight reminded me of something I experienced when my very beloved grandmother (for whom I am named) died. She suffered a lot physically the last few months, and also suffered from dementia (as I know your mom did). My mom was the only child living nearby and had to make the decisions, visit, etc. It was very difficult.
I remember expressing relief when my parents called to tell us that she had died. I knew that her suffering was over and she was in heaven. I didn't shed many tears at that time.
However, something unexpected happened about a year later. I would tear up when I thought about her--when I was doing something she would be interested in, when we visited my family and she wasn't there, when her birthday came around, etc. It was so surprising to me! She had been gone a year--why was I grieving then? I concluded that the remembrances of her last year and especially her last few months had faded, and I was now missing the grandmother that we all knew and loved. It was a strange and unexpected occurrence.
I don't know if your experience will be like that at all, but I thought I would share it.
Your experience was similar in fact to my mine with my grandmother (nan).
For about 2 or 3 years I could not remember her any other way than how she had been at the end but then gradually I started to remember how she was.
Somehow with mum I started remembering her how she was very quickly after she died I think because she had not been at home for the last 18 months whereas nan died at home.