Well, when you play matches ur more nervious and tight as in practice, its normal for everyone even the pro players, so mental toughness for me is when ur in such situations that ur opponent is rly pushing u hard and u somehow can get past the tightness and really focus like crazy, some zen like focus where you probably wouldn't even feel if someone shot you in the leg because ur so focused on the ball and the point.
For me its always like this when I play players better than miself.
I start losing and going down, then when its maybe like 4:1 or 4:0 or something, my mind gets in this super state where it starts focusing insanely, its like all you see is the ball and you don't see or hear anything else, its like ur abilities and perception and mind focus is raised by 200% like some adrenaline rush or something, and you raise ur whole game and everything and give ur opponent a really hard time and get some points back or get close ven if you lose in the end.
You hit such crazy shots and save such crazy balls where before ud make only 1 out of 10 but now seemingly you make all, its really insane and I can't explain to miself how its possible, is it really the adrenaline rush? It just might be... ive read that when ur in that state all ur abilities, perception, focus get much better.
It always seems to happen with me.
My problem is when im better than them and start leading and gaining an advantage, then I get even more nervious of "****ing up the lead" and start playing worse and making more mistakes and being more tight.
Funny thing really.