Reverse the Score
Do you play better when you are behind in the score? Many players do. They relax, take a “nothing to lose” attitude, and focus more on just playing one point at a time.
Think that way when you are winning and trying to close out the match.
In your mind, turn the score around and pretend you are losing. Relax, figure you have nothing to lose by focusing on one point at a time, and regain your confidence.
Congratulations on the comeback!