An established tool that many counselors, myself included use to help others gain a new perspective on their situation is reframing. Psychobabble aside, the principle of reframing is intended not to rationalize away /make excuses for why something happened, but to see the picture from a different angle. After all, what is done is done. How we respond from setbacks and learn is what really counts.
For example: After careful preparation and tapering, a swimmer still does not reach her time goal in the 200 Fly in a swim meet. She exits the water frustrated initially and talks to her coach about her recent experience. "What were the positives, what did you do right"? the coach asks her, then "What was out of your hands"?. Taking an objective stance, she realizes that some things happened that she was not prepared for: she did not get a clean start due to a previous false, and her goggles were not sealed. In the moment, she should have surmounted the obstacles, but in reality, the end result is not always what we planned. Acknowledging the things that happened objectively is a start, and can divert the athlete from the unnecessary negative viewpoint regarding a performance. Mental toughness can include the ability to see things from a different perspective.
This raises the question of mental toughness. What generally comes up in an athlete's mind regarding the subject is the ability to power through in the moment, not let the opponent get inside your head, blocking out pain, blocking out distractions, etc. etc. This is definitely part of the toughness equation. But is there more to it?
Mental toughness can also be extended to how one responds to setbacks and performances one views as sub-par. One can see a poor performance as a setback, being out of shape, or the beginning of a 'funk'. How would a mentally tough athlete approach the situation? Perhaps they would be able to process through a rough practice or competition. Perhaps they would highlight the controllables ("C's") and the uncontrollables ("UC's") of the given event.
A mentally tough athlete can compartmentalize the lack of performance due to difficult weather conditions or stressful background circumstances. A mentally tough athlete is both flexible and forgiving in regards to their performance. A mentally tough athlete is able to reframe when things don't go as planned (i.e. "I did everything I could on that day", "I still have 'x' events this season" etc.).
Thinking "I will move past disappointment" (or a variation of it) may be a good mantra to practice. A mentally tough athlete can think "so what?" after a bad game.
In summary, being tough not only involves being able to block out and gut out variables when competing, it cal also involve accurate appraisal and emotional control following perceived setbacks.