Pitching is a mentally-intensive activity, and requiring zen-like concentration is the key to success. As a pitcher you need to “let go” of everything external to yourself and stay centered in your internal Zone.
Control the variables (mechanics, mental, and physical). Concentration is the key to consistent pitching, and nothing snaps concentration faster than a concession to external factors and letting them bother you.
So what if it’s raining, or windy, or hot, or cold, or the mound is too high or too low? Who cares how many men are on base or how many thousands there are in the stands? What’s the difference between a spring training game and game seven of the World Series? Even the inconsistency of the umpire or his squeezing of the strike zone can be handled by focusing on working with what area you learn that he will give you. Don’t fight your circumstances, accept them for what they are and go to work accordingly.
You can only control yourself. That’s it. It’s not an easy skill to learn because it goes against human nature, but once mastered it is possibly more important than any other skill as a pitcher.
Good Luck and Play Ball!