During a match, many players are thinking about what they cannot do. Instead, you should be thinking about what you can do! Many times in a match, shots that are normally yours will leave you temporarily. When this happens, quit focusing on what you cannot do and start focusing on what you can do.
Example: You usually hit your first serve very well and hit a few aces. It suddenly leaves you. You start getting frustrated and thinking too much about the serve and possibly start to fear double faulting, thus the focus of your game has changed to negative thoughts about your game and your confidence can wane. Solution: Hit your second serve first and get the point started so that you can focus on another shot to win the point.
Example: You usually have a nice consistent full motion serve return and suddenly it leaves you. Solution: Block the serve back like a volley motion. Either block it back low or, in doubles, block up a lob over the opponent's net person.
Example: You usually win because you are a very consistent baseliner. Your ability to do this suddenly stops. Solution: Start forcing yourself to attack the short ball more often and attack the net. This might rattle your opponent until you settle down.
In general, focus on what you can do, not what you cannot do. Tennis players tend to be very intelligent and thus become very good problem solvers. Take the challenge, solve the problem, and prepare a back up plan if the original plan isn't working. Do not limit yourself to relying on one single shot or one strategy. Be flexible!
Good luck on the courts!