How Do you Tell?

My cat is 19 years old. The vet calls her "ancient" and that's not a bad description. She appears to have Feline Cognitive Disorder (kitty Alzheimer's). She has very little body mass (she's skin and bones) despite eating well. She has some pain in her hips that doesn't prohibit mobility but you can see its effects. If she's in deep sleep she sometimes has problems with incontinence. And she doesn't hear, see or smell so well anymore.

She was the first domestic animal I ever... acquired (you don't own cats). I just wonder if I'm hanging on to her for selfish reasons. I just can't tell if her quality of life is to the point that it's better to ease her pain. With the dogs, it was easy. They stopped eating. When they went downhill, they went quick. Nikita has been working toward this point for at least three years.

I remember my mother-in-law's cat, George. I saw him in a similar condition and was shocked because it was such a dramatic change from what he had been. But in my eyes, Nikita has gotten there gradually and, while I notice the difference and it is significant, she still seems like she's hanging on. Obviously, if it had happened overnight, I would know. But how do you tell with a cat?