Training to avoid elementary mistakes long into tournament games? I just got back from a chess tournament after around 5 years without playing tournament chess.  I did mediocre in this tournament, and I attribute it to struggling to maintain concentration after 3+ hours into a game.

I'd play a relatively decent game throughout (imperfections on both sides here or there), but lose e.g. to some simple tactic late in the game.  Something that I would ordinarily see easily, e.g. a knight fork, but was not simply looking for it late in the game.

There seems to be no point in learning openings, endgames, etc. when you end up down a bishop and end up resigning.

Question: What training exercises (drills) can we do to prevent making elementary mistakes long into tournament games?
Fatigue leads to errors of judgment. Fatigue in long games manifests as 'seeing things that aren't there' and 'not seeing things that are there." As stated before, current health status, conditions of the room, even how much sleep was gotten the night before, are factors. Actually, errors in judgment is an integral part of the game, or as one grandmaster said, "whomever makes the last mistake loses."