Anger

I think that the understanding Alois talks about comes with experience. If one trains regularly and with purpose, it will improve their the ability to control negative emotions with time. The reason is that regular training helps to understand the game deeper. If one understands the game better and the reasons why they missed a shot or lost a match, it becomes much easier to acknowledge the miss and accept it. Particularly, some shots that seem to be easy are actually not so easy ones, and it takes experience to acknowledge that. One of the best examples is a high ball overloaded with backspin. For a beginner it seems easy, but in fact it requires very precise technique to play it well. Once one understands this, it is much easier to calmly accept a miss against this shot.

Another thing is understanding how much time and effort it takes to elevate one's technique to the level it works in matches. It is easy to fall into thinking that there is something wrong with them and it is the reason why their shots fail them in matches, hence the anger. But the fact is there is nothing wrong with them and eveybody had to go through this path. Even those super talented juniours that jump around and loop everything, they just played zillions hours of multiball with their coaches. Once one developed a technique to a solid match level (e.g. a forehand loop) it becomes much easier to acknowledge a miss with their other shot (e.g. backhand that is still work in progress) simply because they already know how much drilling it takes for the shot to be reliable. So they know that if they keep training, the time will come, maybe not next month, but next year it will be OK. So there is no reason to be angry.