Quality of Life

I'm getting old and forgetful. I think of my parents who died while still active and only a short illness. My mother-in-law died of Alzheimer's complications after a long decline. I remember when she realized she was losing it and lost her driver's license, went through the mean angry stage, the "I want to die" stage and then "we weren't sure she was there at all stage." I don't think anyone wants to go through that, the problem is when to end it. When I am still able too?
I don't think a living will (DNR) would help. Many people are able to feed themselves and drink long after other higher functions have ended. Quality of life is more important to me than just staying alive.