Try some visualization exercises.
Visualize yourself pitching against each individual hitter on the opposing team. See yourself dominating, spotting up, getting ahead early and finishing strong. When you step on the mound for real, your mind remembers the visualization exercises and thinks it has done it before, this should help with confidence and likely performance.
Before starting an "inning" in your visualizations, say a keyword that helps you "lock-in." If you have seen For Love of the Game, when Costner's character says, "Clear the mechanism," he zones in and forgets everything but the next pitch. If you work on this in your visualizations, when you step on the mound for real, and say your keyword, you should feel a confidence and a higher level of focus.