Remember the Game Plan
Re-focusing on the strategy or game plan throughout the match is another easy method of getting back on track. Focusing on the game plan you and coach have outlined offers a distraction from previous points/games/sets and gives you a future oriented task to move forward with.
Too often, players are so hopped up on previous events that they forget about what they set out to do. If it helps, write down the strategy or performance reminders on a note card and check it every changeover.
Get Back to the Basics
If the match has become stagnant or the air has begun to empty out of your balloon, remember the bread and butter aspects of your game. Hit the shots you feel most confident with at the time and commit yourself to the competition.
As an athlete, competing is a basic instinct, but our minds often distract us from getting back to what we know best. If you catch yourself getting hung up on previous points, fall into a string of unforced errors, or if you feel tight, assess what patterns are working and stick to them.
If your forehand crosscourt has been reliable all day, hit a few more balls in that direction to get into a rhythm and then take the next shot that looks good. You will find your other shots will get better and your confidence will continue to grow.
You might notice one common thread among all these suggestions: each is a controllable factor. Tennis is full of uncontrollable areas, but performance and effort is not one of them. Ultimately, you are the one to decide how to respond to situations faced on the court.
So, with this in mind, continue to monitor your matches to see if you can figure out any common themes that need improvement (i.e., slow starts, inconsistent play when holding a lead, etc.). It will take a level of accountability and deliberate effort to notice when you are falling into your old habits, but fortunately once they are identified you can begin to right the ship.
Good luck this season!