Pitching is either extremely tough, or extremely easy for people. I have played with guys in college and pro ball that seemingly could throw a strike with every pitch whenever they wanted. Others, including me, struggled throwing strikes in low effort bullpens, and got even worse in games. One big cause of this, and it is more than just mental, is that players have been pitching since they were 6 years old, and have never learnt to properly throw a baseball. Learning to throw, through long toss and proper drills, can help eliminate inefficiencies that are magnified on the mound.
The basic idea of throwing is properly timed movements designed to throw a baseball to a target. Pitching is just throwing, but with many other things that make it much more complicated, such as off-speed pitches, location, timing, holding runners, etc. The problem is that kids have never had time to learn to just be a thrower. Even collegiate and professional players will take up to 8 months a year to long toss, build velocity, and get their arm strong before ever getting on a mound. Doing video analysis of previous pitching, and even throwing, can help trainers implement drills that will help the player feel what how they should be throwing. I am a huge supporter of letting the drills do the cueing with players, instead of a ton of external cues that a player may never understand. This is a big reason why I like doing weighted ball training. Weighted balls force your body to eliminate inefficiencies, as the arm cannot throw a heavier ball as easily and with as much force as a baseball if there are weaknesses in the delivery.
Many kids I see from coaching struggle with pitching, although I can tell they have had many, many private pitching lessons. Their mechanics are cookie cut, and they always will ask typical questions, like did I sit back on that one? or did I get out front? The problem with this is that they have never taken the time to learn how to throw, and just end up pitching year-round. I would love to see a grade 10-12 student take a month off from throwing after their summer season, and just spend the next 5-6 months long tossing, and implementing drills with weighted balls, to retrain themselves. If they are especially struggling, I would even recommend taking the entire year off, and making throwing a priority. This only applies if the player has baseball in their plans after high school.
Overall, pitching becomes much more complicated when a player doesn’t know how to throw, and thus it’s incredibly important to take time to relearn throwing.