Have you looked at whether there are times when your capacity for action and/or enjoyment is better? When it's worse? This might give you some hypotheses to test to get some improvement.

I tried to do a somewhat quantified analysis of pairs of years, starting from 2002 (anything before 2002 is distorted by lots of things that I can't change without technology and the rest of the world obliging). I found that the best phases seemed to correlate most strongly with something resembling enforced pomodoros, and something resembling access to people. Trying to artificially recreate these conditions is much harder than it sounds, and I have a sinking suspicion that there are some long-term effects of all this that would make it an uphill battle even then.

\<dietary recommendations\>

Agreed and attempted. Too much sugar and carbs in a short span of time has a horrible effect; smaller amounts of sugar over a longer period seem to help, and there is a mild but noticeable improvement from other types of food, but I've never managed to sort out anything better than "find a way to get away from the concentrated sugars ASAP".