Diana Nyad just did something that no other person — not even a 28-year-old version of herself — has ever done.

She swam from Cuba to Florida without a shark cage, and she did it despite pain, weakness and nearly constant vomiting.

She beat the odds. How?

“It was not just about the physical, but the mental,” Kathryn Olson, CEO of the Women’s Sports Foundation, told NBC News. “I am sure there are faster swimmers out there and stronger swimmers out there.”

“I think it was her belief in herself that she could do this,” Olson added. “She had me convinced she could absolutely do this, no question. That is what made her different. She worked through the pain. She had such a strong belief in herself and her ability that she was able to convince all those around her as well.”

Nyad’s feat proves how much a mental game means to success. It begins with belief.

What, you might ask, do you do when you truly don’t believe?

There are two things.

First, if you don’t believe, just act like you believe. Among the three most powerful words in the English language are “Act as if.” Did you ever see fighters at a weigh-in? They stare at each other, nose to nose. They don’t show any fear. But the key word is “show.” Maybe they don’t show fear, but they probably feel it. Wouldn’t you feel fear if your opponent had the power to knock you unconscious?

The second step in building self-belief is to start winning more. You don’t have to win big victories. You just have to win. You do this by keeping promises to yourself. If you tell yourself that you are going to get out of bed at a certain time, keep that promise. Same thing for promises you make to yourself about exercise, study, diet or anything else. The more often you keep your commitments to yourself, the more you will feel confident about your ability to keep doing so.

Diana Nyad swam 100 miles across open ocean because of her mental game and her self-confidence. Work on yours today.