Whenever a negative thought appears in your mind, tell yourself:

STOP

and turn it into a positive thought. Practice doing this a lot, even outside of tennis, until little by little you no longer need to say STOP! Because immediately you will have a positive reaction. To do this exercise, always start with a deep breath and imagine a positive thought. Remember that thoughts should: be short, be expressed in the present tense, contain the word I, express what you want to do, convey conviction about what you are going to do.

Below is a table with negative thoughts and their transformation into positive phrases.

Negative thoughts  -  Positive thoughts

1. I just got an easy answer wrong, how stupid!  -  I move forward, focused on the next exchange!
2. I got it wrong again.  -  I take a deep breath and look at my opponent.
3. What will the teacher think of me after this umpteenth mistake.  -  I focus on giving it my best shot.
4. I’m too uptight, I’ll definitely make a mistake.  -  I breathe and visualize the serve.
5. I can’t stand his service.  -  I am responsive on my legs.
6. My heart is in my throat and I don’t know what to do.  -  My heart is full of energy, I feel strength.
7. This opponent is too strong.  -  I look at the ball, I hit the ball.
8. I must not lose this set.  -  I imagine myself serving the way I know how.