I think the role of mental toughness, the stubborn refusal to give in to fatigue, disgust or despair, is an underrated part of one's chess results. It's an entirely different subject than chess ability, for any given player may have a lot of one and not much of the other.
Every tournament player in history, right up to world champion, has had the experience of getting a 'won game' and having the 'win' slip away because the opponent stubbornly played the best defense, move after move, until finally the 'win' became a draw or loss in the tournament table. If you study the games the greatest players you'll see how rarely they go down without a hard fight after getting a bad position.
Below that exalted level, I've noticed that most players are unable to 'usefully forget' how the position on the board came about and just play it to the best of their ability. If they made mistakes earlier they find it hard not to let the regret creep in and cause further error; if they've outplayed the opponent they find it hard not to celebrate their good position in advance of the actual resignation...
I have personally been guilty of all these mental errors at one time or another, but I'm striving now to do better, and not to let the current trend at some moment or the other during the game throw me off too much. As I post games and results here I'll try to comment on this aspect and see if I'm succeeding.