A Marine veteran and mental toughness coach shares how a single sentence can change your life

"I'll ask them, 'Who are you?'" Wittman tells Business Insider.

"And they tell me what they do: 'I'm the CEO of a defense contractor firm.' 'I'm the EVP of a pharmaceutical sales company.' 'I'm a cop.' 'I'm a Navy SEAL,'" Whittman says.

But that's not what he's looking for.

"Those are roles you play," Whittman says. They're not who you are.

"If you don't have an internal identity of who you are, based on an internal strength, and something that you aspire to, then when it hits the fan, you have nothing to draw from. You're not rooted. You're not grounded," he says.

The Marine Corps veteran and former US Capitol Police agent protected members of Congress like Hillary Clinton, Nancy Pelosi, and Joe Lieberman during his time as a federal agent. He's now the author of "Ground Zero Leadership: CEO of You" CEO of The Mental Toughness Training Center, where he teaches clients to condense their core identity into an easy-to-remember sentence.

"Once you know who you are, everything becomes easy, and it becomes clear," Wittman says. "And all this other stuff melts away because all I have to do is ask myself, 'Am I acting out who I say I am?'"

For example, Wittman's own personal statement is that he's "a man of excellence who always keeps his word."

Wittman considers that a brief summation of his bedrock identity, whether he's acting as a CEO, a husband, a father, a Little League coach, or a friend. It's not really an elevator pitch; more like a stripped down affirmation that one should seek to live up to.

"That's who I am," he says. "Once I have that established, then whatever storms of life may come, when I need mental toughness, when I need to be under control under pressure, then I'm not worried about all of those things. Suddenly, I don't fear failure either."

Wittman says that humans tend to fear rejection and crave acceptance. Creating a concrete summary of your identity can really help you accept yourself — and thus accept others. So outlining your core values and identity can encourage mental toughness in the long run.

"If I didn't have a $100 bill, I couldn't give you a $100 bill," he says. "If you haven't accepted yourself internally and you don't know who you are, how could you fulfill everyone else's number one need — acceptance? If you're looking for acceptance externally and you haven't accepted yourself, then rejection will always be a problem for you."