Oh Jan,

I lost my mum 13 years ago to bowel cancer when I had a 3 yo and an almost 1 yo. (Dad died from lung cancer when I was in my final year of high school, so mum did some of my *tough* years single handed!) How much I have missed her as a friend, sounding board and mentor! She was a wonderful grandmother to my older brother and sister's kids who lived closer and I'm saddened that my own kids didn't know her well.
I know how you must be feeling - it is hard to even start to repay the debt of gratitude we have for our mothers if they have become one of our friends as adults...
Spend as much time as you can with your mum, and remember to include your DD in her life, even though it is sad to see - she will process grandma's changing state in her own way, and it will help her to understand that the bond kids have with their mums never stops.
No matter how old we are we still always need our mums, so my heart goes out to you.