The difficulty is that at our age we are (or at least I am) beginning to be more and more aware of our gradually increasing physical limitations, some at a greater speed than others. And then we/I start thinking about all the things I used to be able to do easily but can no longer - at least not without difficulty.
So.....what to do? Feel regretful/angry/sad? - or find those things that you can still do well and concentrate on those, or find new things to enjoy.
I am not as fit as I was and my regret is that when I was fit I did not really appreciate it - did not realise how quickly it would fly away!
But.......I am able to do lots of things now that I enjoy, and some things that I would never have had time for previously. They are things that do not require physical fitness: quilting, reading, singing, running a singing group, reading to my DGC etc.etc.
We have all done things or not done things that we regret - but life and we are not perfect - no-one is, no life is. Please come down from "up the wall" Teetime and just remember that other people look as if they have perfect lives and that only you have messed up - but we are all in the same boat without a doubt, so there is strength in numbers!!
The people who do best in old age are those who keep an active interest in the world, be it learning a language (my FIL was learning Spanish at the age of 92!), reading all those books you did not have time for before, vegging out in front of daytime TV - whatever ticks boxes for you. D them with relish!
You are not alone!!! 