For a while, I had less mental energy than I used to. This used to be infinite, but it got harder to do things and I would get mentally exhausted after a lot of focus, which I can observe is fairly common but never used to happen to me. I talked to doctors, etc., about this and the universal response was basically "you're getting older; it happens", until I paid some "overpriced" private doctor (this is Canada, normal doctors are free) who did some tests and found a trivially fixable problem.