When your energy is clamped, you won’t be able to produce a smooth movement. No smooth movement - no stable play. No stable play - no stable outcome.
Removing tension is one of the hardest things to do while learning some technique. For example, if in table tennis your hand is clamped, if you gave too much tension to it, you won’t be able to “let it fly”, you won’t be able to give a quality spin to the ball.
Tensioned movement also block fast position change, block productive footwork. You may know well how to act already, but you body may be disobedient to your desired course of action.
Why tension happen? Lacking of experience (you don’t know what awaits you), psychological pressure (you care too much of the result, opponent seems stronger, etc.).
Try to start playing for the sake of process, not the result. Love your game, have passion to research it, despite the result of winning or losing. In fact, nothing is pushing you as you think. Relax and get experience. Relax and apply experience. If you master proper relaxation you will conquer many new heights in your sport adventure. Accept wins and feel sense of progress. Analyze and let go of bitter loses. Journey will matter most, and results will just serve as temporary checkpoints.