That's worth a dollar right there!!
It happens to a lot of players, from league and recreational players, to even some pros occasionally. If you bring your goals and ego INTO a match, I.E. being obsessed with beating a particular player, the score of the match, the importance of it, etc. these things detract from focusing on your process.
We can’t live in the past or the future, only in the moment we are in at any given time. So you can’t abide letting your mind become preoccupied with past failures, or missed shots, or the importance of outcome of the match you’re in, who’s watching, what they think, etc. as these things detract from you giving your full focus to your process, and the shot currently in front of you. Failure to let all that other stuff go, so you can give your full attention to the present moment, never leads to anything good.
I remember overhearing a conversation where one person commented to his player that he seemed tense. The player replied that his family was in the audience and he was concerned of what they’d think if he lost. His sponsor said, well unless you change your line of thinking, you’re going to find out what they think, because you’re not going to play your best when distracted by something you can’t control.