I don't remember who is credited with saying it before Clint Eastwood, but one has to know their limitations. I believe it is fine to know your comfort zone and if opportunities arise that allow you to enlarge it a bit and you are able to take them, that is perfectly fine. If one attempts to do something or yearn for something that is simply no longer possible, then it may be time to mourn that friend and move on. Before I was diagnosed with chronic pancreatitis, I was at the gym every morning at 5AM for 90 minutes and no longer am able to do that and I miss the workouts, and the friends I had there, but it is simply not going to happen in the same way again. I still try to workout at home when I am able and motivated, but that leaves out the friends part, but it is simply what I am able to do and I accept that. I don't think that is the same as "giving up".