God this thread is terrifying. I hate "forgot the words syndrome." So annoying to have song down pat guitar and singing an hour before actually performing it, only to find yourself mixing up chords and verses.
Then the shaking starts and fingers keep missing the proper strings and voice weakens and starts to tremble.
Well, one thing about aging, though it may affect the memory and the skill, the experience that comes with age teaches you how to forgive yourself.

What can one do? Just keep getting back on that stallion song. Maybe you will never break it, but hell ain't it a great ride?