She may be sleeping a lot because as cats get older they seem to "rest" more. My best advice is to take her to the vet and have her checked out. If there is a reason other than age he could put her down. Otherwise enjoy her last days or months. As long as she can get around and feed herself she will go when nature says it is time to go, again, unless she is ill and suffering.
My cat died only a week ago and he was put to sleep my cat only had 6 months to live and very ill. After I had him put to sleep I felt very guilty but I know I did the right thing. You do what you think is right.
Our cat had a stroke, and I hand fed her with a squart bottle for 1 week before she passed away. We were all there when she breathed her last breath. If you could only see the look in her eyes after I fed her with the squart bottle, you would never even think of taking her to get euthanized.
I would hope by now you have humanely put your cat down. Please rest assured you have done the right thing!
I can understand your situation. Your cat is old, and from where I see she has no chances of recovery. I believe Euthanizing is the best option available with you. This would put an end to your cat's misery and even yours. As you are suffering with it right now. So Plan a Euthanasia with your vet and be there for her when the end comes.