Loosing a game to a cheater.
It's ok if the game had equal chances and you lost, but there are certain games where you did almost nothing wrong, yet the opponent somehow managed to accumulate those +0.03, +0.11 advantages slowly strangulating you until somewhere in the late middle game you loose a pawn, which is methodically squeezed through in the long and hopeless end game.
Such games are impossibly taxing and ultimately frustrating.
Finding the proper guidance for seeking to improve my skill in making the vital decisions in end game phases is to me the hardest thing.