The advice I can offer is in three parts.
First during the tourmanent all your focus must be on the future moves of the game in progress. Previous games don't exist. Future games don't exist. Previous moves don't exist. It doesn't matter if you lost or won all previous games of the tournament. It does not matter if you made a mistake in the game which resulted in a weaker position. It does not matter if opponent made a mistake which you didn't exploited. Keep all your focus on the moves you are about to play. This way you will almost always play to the best of your abilities.
After the tournament analize all your games. Try to find inaccuracies in all your games. I have a philosophy than every game I lost, I lost because I made a mistake. If you play the best possible move every time you can never lose. If your opponent does the same game is always a draw. Usually in chess the player that lost did so because he/she made a mistake the opponent exploited. But just because you won does not mean you played flawlessly. Most probably your opponent did not saw your mistakes. So you need to find them and correct them.
And third part is no matter how good you are you will lose. You can never change this fact. No matter what you do, no matter how good you become bad days happen. Oversights happen. Just make peace with yourself that losses will happen and try to learn from them.
I wish you best of luck.
You could be a Stoic. But who are we looking for??