There are ways in which this part of the change seems the most natural thing in the world to me, which isn’t to say it isn’t painful and distressing. And I’ve noticed the experiences you describe in myself, too. (I’m 61.)
A couple of things spring to mind. One is that obviously it’s physically/physiologically impossible to remain as strong and vigorous as we were — all those 40- and 50-year old longevity experts notwithstanding (i.e. wait till they hit 65 or 75!) — and that’s a hard adjustment to make. Another is that, well, I think it might be called ‘wisdom’, that thing where you realise a lot of what you do doesn’t make a blind bit of difference to the world. You realise that the world you can actually affect is a lot smaller than the one you thought you could affect when you were younger. (Me anyway.) And this can be a direct road to ‘why bother’. Again, it makes sense.
All I’m saying is, a lot of this actually makes sense. How to change it is another question. I’ve been trying to change how I think about myself, see myself, be less judgy re my state of mind, relax into it, even sometimes enjoy not wanting to do anything but lay about. A new approach for a new person, given that the old one is not coming back. It’s a work in progress…
Not saying it’s the answer, or possibly any kind of answer. Just throwing it out there for contemplation or something.