I jumped from the first page to the fourth. You already have the right answer over and over, breathing is the key. Slow deep breathing lowers your emotional temperature. On the other hand when it is five in the morning and the rooster is crowing and you are sitting on the hot seat but flat footed as hell deliberately breathing faster and a bit shallower can get the blood to flowing.
You can read books by the dozen and go to zen coaches but the answer is as simple as breathing. Your breathing changes with different emotional states but it's a loop, you can change your emotional state with your breathing.
It is also good to stop and think. If it is a small tournament with a few dozen people or a big event with a couple hundred people, even thousands at a few events in Vegas, there is only going to be one winner. Every other person there will be a loser. Don't feel like you are the Lone Ranger!
One thing that can help you a lot is never talk negatively about yourself. You came planning to win the event, own up to it. If you can't make yourself admit to others that you came to win at least say something neutral like Fat Albert, "Hey, hey, hey, I am here to play!" Don't plan to lose, don't hang with losers with no chance of winning, don't say or listen to a lot of negative talk, it is contagious.
First time I went to a big invitational with people from other countries and all over the US I was a bit intimidated. I cruised the crowds and found that only maybe thirty came to win the rest were just there to enjoy the experience of being in the event. When I realized that three-fourths of the field were just cannon fodder I was OK. I competed with thirty people or more most weekends.
Confidence is key. You don't have to be loud and ugly about it but you came to win. Telling yourself anything else is inviting nerves. Something else I do that works for me is flipping things around. Most look at some of the best in the world and figure they are the pace setters that have to be beaten. I always see myself as the pace setter. I am laying down my best performance possible, everybody else has to try to outperform me. It's a mental thing, but we have to win the mental game before we have a shot at winning the physical one.
While talking breathing, do it! Some people hold their breath while shooting. Sometimes we take a few extra practice strokes and find ourselves short on air by the time we are taking our final stroke. Get up off of the shot, now when you get back down breath shallowly but keep breathing so if the shot takes an extra fifteen or thirty seconds you still have oxygen. Never try to shoot when you need to breathe!
Works for me, everyone's mileage may vary.
Hu