You need to ask yourself if the idea of a competition is a stressful experience or an exciting experience. Psychology and physiology are connected. Under a stress or threat condition, you might think “Everyone is waiting for me to fail. I am not prepared.” There are physiological changes that occur. Your veins contract and your blood pressure rises. You burn out the glucose that is circulating. You might feel tightness in your lungs. In contrast, when you are in a challenge state, your veins dilate. You burn glucose stored in fat cells, which means you have longer sustained energy. In a challenge state, two extra liters per blood can pump per minute. In chess, that means that more oxygenated blood to your brain.
As much as you are practicing it will be different in competition. If you are stressed about tournaments, you might want to play MORE tournaments to get used to tournaments. That’s called the stress inoculation model. You will build stress immunity by playing in competitions. If you are excited about competitions, make sure that your testosterone peaks at the right time. You want to be revved up and challenged and excited but not too early – it's about timing.
Ask yourself, “Is a particular match a threat or a challenge?” Be mindful of your state of mind entering the game and during the game. Even when you are winning, the fear of screwing up may make you more fixated on an error when you need to move on to the next move or the next game.