Every player knows the feeling: the game is going fine, then one mistake turns into three, and suddenly you are playing against yourself as much as the other side.
The fix is boring and it works. A short routine before every point or every serve — a breath, a bounce, a look at the target — gives your mind somewhere to go other than the last mistake.
And remember why you booked the slot. Nobody is watching the scoreboard as closely as you are. The evening was for playing, and the score is just what happened while you did.