Usually when I hear about psychology in chess from popular books or movies, they get it all wrong.
First it's important to mention that when two very experienced players play each other, they both know (in general terms) what each side wants to do. They know which pieces are the best and in general how the opponent will try to use them. The skill is in who can do it better. For examples lets say you and I had to paint the same scene, we both may know what colors or techniques the other will use, but one of us may be much better than the other in completing this task.
The psychology in chess has more to do with opening choice and body language at the board. For example if you know your opponent has a favorite defense as black, and you are playing black against him today, you may try to use his favorite defense against him. Or maybe you are a player known to favor a certain opening attack, so you spend a long time preparing a completely different opening to surprise your opponent.
As for body language I've heard top players say they pay attention to, for example, their opponent's breathing. If a player is excited or nervous, their breath will be faster and more shallow. In this way a player might realize what their opponent is planning or how they evaluate their odds.
How many different psychological mistakes can a player commit during a game? Or there is no psychology at all?