If she has dementia and hasn't been able to remember you for a while you may have already mourned her loss. I agree with everyone else there have been times in my life when loved ones quality of life was so poor that I knew they were in a better place. I'm a Christian so it was somewhat comforting to know they weren't suffering any longer.

I'm so sorry for your loss. Having a relationship with a grandparent all the way up into adulthood is very special.