Focus on what you CAN do
Many good comments in this thread. My suggestion is: try to stop focusing on what you CAN’T do, and instead focus on what you CAN do.
I had planned that when I retired I would work on Habitat for Humanity, because I really enjoyed that kind of work. Now, that is totally out of the question.
But, I have become the treasurer for a small 501(c)(3), something I can do sitting down. I also volunteer at our library, researching the value of gift books – again, something that does not require physical labor. I have also taken a couple of stained glass classes, which I have always wanted to do. Working with glass requires some amount of standing, but some of it can also be done sitting down. So, I fixed up a work bench at which I can sit on a high stool. I still can’t do it for as long as I would like, but I can do some of it.
I still miss long walks, and working in the yard, and the things I can do on vacation are pretty limited. But, there are still things I enjoy, and I intend to get as much pleasure out of them as I can.
So, try to find things that you are still capable of doing, and concentrate on those things. You will find that life can be more pleasant and rewarding than you might have thought.