As long as I've been around baseball I've been able to understand the law of averages. That has helped me to stay focused on the fact that in baseball there are simply more opportunities for chance to play a role in the outcome of any game. Because of the many external factors that affect baseball, even the best teams will struggle to win 66% of their games. When it comes specifically to pitching, a pitcher can increase their confidence just by understanding that even the best hitters will make outs 6 1/2 times out of 10. When a pitcher focuses on the 65% chance of getting an out as opposed to the 35% chance of a hit, they become more confident.
Far too often a pitcher believes that if they make the appropriate pitch with the appropriate speed, movement and location it will work 100% of the time. Sometimes, even to my own dismay, the hitter wins the battle. In the instances when you did exactly what you wanted to do and the hitter still beat you, you must simply tip your cap and move on to the next batter. You did your job and you have nothing to feel bad about.
A pitcher must focus on their job and believe that over the course of a season the good-pitch flair hits will equal the poor-pitch line drives that are caught. Even the umpire calls that drive us crazy will even out over a long season. Weather, run support, bad hops and great plays will also even out over the course of a season. When a pitcher begins to understand the law of averages they will then begin to focus or refocus on their job of pitching and let the rest take care of itself.
For those pitchers who are afraid to challenge hitters because they don't understand the percentages, have them chart batting practice. A batting-practice pitcher throws pitches right down the middle at greatly reduced speeds with little or no movement and hitters still make outs 50% of the time. A young pitcher sitting behind the pitching screen watching batters get themselves out will grow in confidence. The pitcher can also learn to read swings and look for hitters' weaknesses. This is a great teaching tool for any coach and learning experience for the pitcher.
The bottom line is that a successful pitcher always gauges their success on one thing: whether they made the pitch, not the result of the pitch.
Work on it!