When you sit down at the board and start a new chess game, you become your board self. Your board self is the personality you become at the board. You have different thinking patterns compared to when you are training chess or watching others play. I think this happens to everybody. It is important to notice this process and to think what consequences it can have.
Because no matter how much you study chess, your board self will have a big influence on how every new chess game will go. Developing your board self is a very complicated process and in fact I cannot say step by step what has to be done. How you are at the board is a personal thing and you have your advantages and disadvantages compared to the opponent's board self. Maybe you get into time trouble before move 40, that is part of your board self. Maybe the opponent calculates variations all the time often when the position requires other types of thinking, that is the opponent's board self.
Your board self is who you are at the board and what you are thinking about and what decisions you are making. These things are connected with each other. Everything you learned might fly out of your head once the clock starts ticking, and if it does, it is because your board self acts this way. To apply your know how to your actual moves in the game is extremely difficult. You are bombarded with new ideas and thoughts in your head. You have to find sense in the symphony of your thoughts. You can try talking to yourself in the head while playing and try to improve this conversation from one game to the next.