Are you choking or forcing yourself to play poorly? You might be making your matches too personal. Are you taking time and watching you opponent, making a lot of eye contact? This is an inefficient waste of your time. In order to maintain your focus, try this in your next tournament or pressure match:
When the ball is coming off your opponent's racket, watch it as long as you can. I hope that you will watch it long enough so that it is very close to actual contact. After you hit the ball, watch the racket of your opponent. This will give you a clue as to what type of shot that might be coming back (for example, a lob vs. a drop shot).
On the change over, try to distance yourself from your opponent. Many players will try to "psyche" you out by talking to you. You can better use this time to gather your thoughts and think about you next game. Take a quick mental check of what you are doing well, where are his/her weaknesses, making plan of attack. In general, unlike normal non-tennis situations where good eye contact is important, in a match it is not important and can be a distraction. Your job in any match is to concentrate and focus on the match. You will have plenty of time to be sociable afterwards.
This technique is very helpful when you are competing or playing challenges against a player that you might be friends with off the court, of if they are a player that you know will try gamesmanship in order to win. Watching the pros, you can see that they do not communicate during the switch and many are great friends that have to play each other all the time. Watching them play their matches and how they conduct themselves in pressure situations can be helpful for you to see how well they are able to center themselves and get their focus back. The sooner they are able to let go of a bad line call or an obnoxious player, the better they play.
I see this happen a lot in tournament situations and often times afterwards the player that should have won the match will lose it. I can see how a couple of distractions will change their winning game into a losing one because they are unable to regain their focus and after giving into the distractions. Then it can mushroom and get out of control so that they have gotten so far behind by the time they are able to get back in the match it has already slipped away. The loss is harder to take than most because the player realizes too late that they should have won. The player does not understand that they had given up control of the game to their opponent by letting the opponent get the best of them mentally.
See you on the courts!