Many players will have a very hard time maintaining their focus in a match, especially between points. Sometimes, they will allow negative thoughts to enter their minds. However, just remember how important keeping the score is in a match. Try this - "total recall."
In a game, for example, keep track of how every point was won or lost. So, if a conflict arises concerning the score, you will be able to settle the conflict early. Example: I won the first point with an ace, you won the second point with a drop shot, I won the third point with an overhead, etc. At first you might think you cannot keep track of this. But you can!
Just practice doing it! Do not give up! You will find that this "total recall" will force you to concentrate better, not allowing negative thought to enter your brain.
Also, practice "total recall" of who won each game in the set.
Good luck on the courts!