Colin,
I have thought about your prior post for a long time and now see this one.
I understand the depression that comes from being a sole care-giver. I went through a shorter version of your experience as my mother's care-giver. Even that experience, which I acknowledge to have been shorter and perhaps less intense than yours, was enough to bring me to a point of not wanting to go on. I knew when and how I would do it, and why. Fortunately, in my case I found a good therapist who helped me climb out of a seriously dark depression. I see that depression in you and hope that you would consider that checking out prematurely isn't always a good answer.
On the other hand, I am reminded of the State of Oregon's Supreme Court, when they were hearing a challenge to the state's "Death with Dignity" law that allowed for very narrowly constructed circumstances when a doctor could legally assist a patient with drugs to terminate that patient's life. I cannot find the original reference, but I can paraphrase.
We in the United States have a tradition of recognizing everyone's right to life, liberty, and the pursuit of happiness. However, when the pursuit of happiness and liberty have become unattainable to the point that a person's life has neither liberty nor happiness, can we reasonably intervene by preventing a person to end that torturous and painful life?
The judge in question was more eloquent than I and surely said a lot more on the topic. Still, it is applicable to the discussion.
Col, I want to treat you with respect on this subject because I understand it better than some. I know we are sometimes antagonistic and I will not presume to be a close friend, but in this we are closer than we might have been on any other topic. If you really want to check out, I can only tell you that it is your life, to do with as you will. But I have to also say in nearly the same breath that your loss of happiness is the depression speaking to you - and you are listening.
Depression can hit any of us and I do not intend for even a moment to deny its awful power, having felt it myself. The only way out is to try to look rationally at your life to see if you have other issues that might offer you some enjoyment in your later years.
I will add that depression can not only make you feel like Hell, but can also make you feel that the search for future enjoyment is doomed before you start. And understand this: If you have a terminal disease like stage 4 metasticized cancer or something of that ilk, I could easily understand your belief that there was no hope remaining. But if your health is merely declining with age (like mine), there is still a chance to enjoy your later years. You won't tap dance or do Olympic floor exercises like you once did, but you can still enjoy performances or recordings. You can still enjoy being with people. It CAN happen, Col. But it can't happen unless you let it, and your depression will fight you on this.
Allow me this familiarity just once: My friend, I sincerely hope that you can find a way out of the descending spiral of depression. It ain't easy, I know. But it IS possible - and I am living proof thereof. Best wishes, Col.