David Goggins, 44, is the only member of the U.S. armed forces ever to complete training as a Navy SEAL, Army Ranger and Air Force Tactical Air Controller. He is also a former Guinness World Record holder for completing 4,030 pull-ups in 17 hours, and one of the world's top endurance athletes, having competed in more than 60 ultra-marathons, triathlons and ultra-triathlons. Recently, the retired SEAL became the New York Times best-selling author of "Can't Hurt Me." Before any of these achievements, though, Goggins was making $1,000 a month working as an exterminator and living paycheck-to-paycheck. He was also out of shape, weighing nearly 300 pounds. Every day, at the end of his 11 p.m. to 7 a.m. shift, he would stop at Steak 'n Shake: "I would go in and get a large chocolate milkshake," Goggins tells CNBC Make It. "And then I would go across the street to 7-Eleven, get a box of mini donuts." On his 45 minute commute home, he'd "pop donuts like Tic Tacs."

As soon as he got home, he'd turn on the TV and blast the volume so he could hear it while he showered. That was his routine up until one morning, when he heard a program about Navy SEAL training, he recalls: "Here I am listening to the TV as I'm showering and, lo and behold, this particular day, I started hearing: 'Navy SEALs. Toughest training.' I was hearing it cut out between the water hitting my ears." He got out of the shower, watched the remainder of the episode and that's when, he says, "I got sick of being haunted by being nobody." He realized that "I didn't want to sit back and continually watch these shows about great people doing amazing things. I wanted that feeling in my head that I believed that they had: of true accomplishment." Goggins, who was 24 years old at the time, decided that he wanted to become a SEAL himself. He had experience in the military — he'd been in the Air Force from age 19 to 23 — but enlisting as a SEAL requires completing one of the military's most grueling training programs.

I got sick of being haunted by being nobody. David Goggins retired Navy SEAL

The day he saw the SEAL program on TV, "I started calling recruiters up," says Goggins. For two weeks, they "laughed at me," he adds. "I heard so many 'No's." Eventually, one recruiter told him to come into the office. Goggins showed up and learned that just to apply to be a SEAL, he had to meet basic physical fitness requirements. At 6'1", he couldn't weigh more than 191 pounds. That meant shedding 106 pounds. He established a training plan and "went on this crazy, crazy, crazy routine, eating hardly nothing." In less than three months, he lost the weight. But that was the beginning. From there, he had to complete SEAL training, which can take as many as 30 months.

Goggins during Hell Week Source: David Goggins

The toughest mental and physical challenge of the program is Hell Week, which is 130 hours of continuous training. Goggins had to go through three Hell Week's in a year, after pneumonia and stress fractures forced him to drop out of the first two. He passed on his third attempt. Goggins credits his transformation to his mindset. The most successful individuals are "focused and obsessed," he says. "They're driven on whatever it is that they want to accomplish in life." He also developed a strategy for becoming the person he wanted to be: "I invented this thing called the accountability mirror. … I would look in the mirror and call myself out: 'I'm afraid of this. I'm afraid of that.'" Naming his fears helped him get past them.