Fortunately, not long after my son starting pitching he was extremely candid in some of our post-game discussions. I quickly realized that confidence is huge in pitching. Seems to be a greater factor in hitting than in fielding.
Since that time I always worked very hard to help my pitchers build confidence. Lots of positive reinforcement, and choosing my words very carefully when things were not going well.
At this point, my son hopes to go on the hill when it is late in the game with bases loaded and no outs. He just loves those situations. And he has become very successful in those situations. He was the top closer on his select team, even though he was 1-2 years younger than the rest of the team. And not a big fellow either.
With regard to what goes on in warmups, it doesn't shake his confidence. If a given pitch isn't working well in the pen, he will go ahead & try it in the game. If it still isn't working, he will use other pitches to get the job done. He throws more different pitches than the average pitcher, so this doesn't pose the problem that it would for a two or three pitch pitcher.
Now if I can just get more of that confidence to rub off on his hitting...