Pool is a very control oriented game. Whether you're playing an offensive or defensive shot the whole objective is usually to stay in control of the table. That being said, sometimes the thing that makes us play great pool is also the thing that makes us fall apart. Our minds start racing about how we will play, how to win the match, or whether our opponent will play too well for us to beat them. These thoughts are completely normal, but to be a great player you have to eliminate the thoughts about the things you can't control and focus on the things that you can. Here are some examples:
1. You're at the table and your heart starts pounding and arm starts shaking. You can't control this 'fight or flight' reaction because it's an automatic response to stressful situations. What you can control is the effect it has on your focus. First, slow yourself down. Panic will make you rush, so be extra careful through your preshot routine. Second, steady your eyes. Many times when you're feeling panic your eyes will jump around. This response would probably serve you well in a true danger situation, allowing you to take in a lot of information in a short period of time, but at the pool table, it is a killer. Force yourself to keep your eye on the ball until the shot is complete.
2. Your opponent breaks in the 9, or misses and leaves you safe. Luck is a very frustrating part of pocket billiards games, and if you dwell on your opponents rolls it can affect your performance, and make you lose enjoyment of the game. You can't control your opponents lucky rolls, but you can minimize his opportunity to get them. By playing a table control oriented game you limit your opponents chances. Focus on not going for shots that are low percentage, and play safe when you're not certain to make the ball.
3. Your opponent is playing perfect pool and every time you come to the table you have to kick. How your opponent plays is just one more thing that you have no control over. What you can do, however, is put the maximum effort into every opportunity you do get. If you give yourself a good chance to get back control of the table sometimes it can create a momentum shift.
These are just a few examples of directing your focus to things within your control. As a match progresses it's important to monitor your thoughts, and eliminate the one's that do you no good. Focusing on how to win the match won't get the job done, but if you continue to focus on the task at hand, winning should follow. You can't control how your opponent will play, or the luck they will get, or the equipment you may be faced with, but if you concentrate on the things you can control, your game and your sanity will stay intact.