Mental toughness is a phrase bandied about rather liberally. I have had many athletes come to me with a reputation of poor mental concentration. In every case I found it was not a mental problem, but a practice problem.
If you are not logging your hours on the court, you will not be performing at the level you are capable of. At crunch time, the automatic response needs rehearsal. So it is not a mental thing, it is a practice thing. I was in the Air Force during the Cold War days. In almost four years I used my military training in maybe three situations of crisis. At that time I did not think because the training kicked in. (disclaimer here: it was not combat!)
The military practices maneuvers over and over so when it happens there is no thinking about it, but an immediate response without thinking. It is automatic. This is the same in our sport. Rehearse to go from worst to first!