“a larger proportion of who and what you are is already behind you, fixed and unchangeable”
Nothing that actually exists for you today is fixed. Nothing is unchangeable. Your quote and my statements, as for all great truths, are equally true. I’d recommend living the truth of my statements over yours, because it is less constraining and less likely to lead to Depression. The big D, much worse than the big C since it steals your life immediately and directly.
“the spectre of your own mortality looms larger”.
That is just an idea. Your actual death doesn’t exist, for you. You can’t show it to me now. You won’t be able to show it to me after your death. And you won’t be who you think you are now at the moment of death, so you can’t show it to me then. Your own death to you is a mere idea. I don’t recommend becoming obsessed with it. See “the big D” above.
“Some time and space for reflection might be called for.”
A very “heady” approach. There is also meditation, prayer, singing, loving, reveling in the beauties around us and within us, or just planting a few seeds and watching them grow. Painting, sculpting, building a deck, helping a neighbor, hugging your child. Dancing with your mop or broom. Knitting a small car out of steel wool. Sometimes getting out of your head and gaining a much-needed distance from your many thoughts is the better thing.
We don’t know where the original poster is coming from, so I decided to throw out some more options. Your approach for yourself is a “flow”-based approach with some observing (which could be misinterpreted as thinking) elements. The entire catastrophe of human existence is a lot broader than that...we have a lot more doors we can open.