Sorry to break the news like this. You are not a robot!
Your body has its needs. You need good sleep at night. You need food and drink. You need to breathe fresh air. You need to be dressed in a way so that your body has the right temperature. You need to feel good emotionally. All these things decide how much nerves you have for a chess game. Preparing for an important game is more than checking opening variations. It is about taking care of your body and preparing physically for the game. Bring extra clothes. Get good sleep. Bring some drink and food to the board. Keep your mind clear. Give yourself an hour without looking at a chess board before the game. Be in your peak performance zone. Feel great. And then go ahead and play your game!