Trudging up a brier-covered mountain in freezing temperatures with a dying headlamp, Nickademus de la Rosa knew his attempt to finish the Barkley Marathons, a 100-plus-mile race in Tennessee, was coming to an end, as it eventually would for most other entrants. The race has no trail markers, an elevation gain comparable to climbing Everest twice from sea level and a finish rate that hovers around 1 percent.

Earlier in de la Rosa’s career as an ultramarathon runner, he most likely would have been stricken with an overwhelming sense of worthlessness and shame for not completing a race. But in the Tennessee woods in March, he saw the upside.

“Instead of hitting myself and telling myself how worthless I am, I congratulated myself on what I was able to accomplish,” he said. “I realized I did not have anything to prove at Barkley. I had no more demons to slay and I was happy to finish early and spend time with my wife.”

It was a significant moment for de la Rosa, who has been grappling with a serious mental illness that has imperiled both his running career and his life.

In a sport dominated by people in their late 20s, 30s and even 40s, de la Rosa was a prodigy. At 19, he finished Badwater, an infamous 135-mile race across Death Valley in California in the brutal heat of July. When he was 21, he completed 135 miles in Minnesota with temperatures of minus 35 degrees. The next year, he became only the 13th person to finish Barkley since it began in 1986. And at 24 he placed second at Tor des Géants, a 205-mile race through the Alps. During that 76-hour race, he slept less than two hours and hallucinated that his running partner’s intestines were hanging out of his body.

De la Rosa said he always ran races to win them, but he now realizes that his motivations were more complex. He spent much of his youth and young adulthood in emotional turmoil, and instead of seeking treatment, he essentially self-medicated by keeping a brutal training schedule and participating in some of the world’s most grueling races. In 2019, at 29, he was diagnosed with borderline personality disorder, which can cause sudden shifts from intense sadness to deep fear to shame or joy. Those with the condition often have an unstable sense of self and struggle to keep jobs or maintain relationships, and many, including de la Rosa, attempt suicide.

The illness affects about 14 million Americans, according to the National Education Alliance for Borderline Personality Disorder. That’s twice the number of people who have Alzheimer’s disease and nearly the same number as schizophrenia and bipolar disorder patients combined.

On a Monday morning in May, de la Rosa, 33, and his wife, Jade Belzberg, 31, sat at their favorite cafe in San Luis Obispo, Calif., where they live. Belzberg is a formidable ultramarathon runner herself, and they’d spent the weekend running in the mountains. The miles showed in their sun-tanned faces, weary eyes and the slowness in their steps.

Over coffee and tea, the couple talked about de la Rosa’s mental illness, their athletic accomplishments and their future, which they see as closely connected.

De la Rosa is tall and broad-shouldered, with unkempt hair and freckles that bring a boyishness to his face. He said his mental illness was both a strength and a crutch. “It was a superpower in races like Barkley that required gritting it out and going into the storm where any idiot would stop because the conditions were terrible,” de la Rosa said. “But this special idiot, because he has B.P.D., would need validation because this win means so much to me, I will push harder than anyone else.”

Like many people who have borderline personality disorder, de la Rosa finds it hard to regulate his emotions. He explained the intensity of his feelings on a scale of one going up to 10. When he tips over a seven, he said, his fight-or-flight response is triggered, and he spirals into suicidal ideations, rage or intense self-loathing. Fears of abandonment and of rejection are two of his strongest triggers. As de la Rosa’s career has stalled, Belzberg, who had not run in a race longer than 10 kilometers when the couple met a decade ago, has taken off. When she passed him on a recent run, he responded by hitting himself in the head. He said all of this in a matter-of-fact way that would be easy to overlook if he were not talking about self-harm.

Dr. Peter Attia, a physician and author of “Outlive: The Science & Art of Longevity,” said he suspects that dopamine, endorphins, a need for distractions, an urge to self-punish and a longing for self-esteem are among the reasons some people with mental illness, addiction and trauma are attracted to endurance sports.

De la Rosa, who moved to San Diego with his mother after his parents’ divorce and said he could trace his unhealthy relationship with running to his teenage years, agreed. “I wasn’t that good at cross-country in high school and was not going to stand out. And then I did a marathon and everyone was like, ‘Holy crap, you did a marathon!’” he said. As someone who felt worthless and struggled to find his identity, he found all of his self-worth in ultrarunning.

In late 2017, de la Rosa was diagnosed with a heart condition that could have been fatal if unaddressed. He had successful open-heart surgery but later developed pericarditis, a condition that inflames the tissue around the heart. Unable to train or race at the level he was accustomed to and with his running career in limbo, de la Rosa spiraled out of control. During a run in British Columbia a few months after his surgery, Belzberg was concerned about the worsening weather and wanted to turn back. De la Rosa said he got extremely angry, shoved his wife in the snow and threatened to push her off the mountain. Immediately overcome with shame and horror, he looked for a cliff to jump off. On the way down the mountain, Belzberg said, her husband alternated between “crying, screaming and laughing maniacally.”

In 2019, de la Rosa shocked the ultrarunning community when he posted on Instagram that he was on high-risk suicide watch. He shared his diagnosis and mostly stepped back from intense training and competition.

Belzberg, who is now a sponsored runner and who represented Canada in the world mountain and trail running championships in June, is thin, with long, dark, wavy hair and eyes the color of Arctic ice. When she smiles, her entire face scrunches. She said there were signs long before de la Rosa’s heart surgery that he was dealing with mental illness. When he could not run because of a knee injury, he tried to drown himself.

Belzberg, who started seeing a therapist after her husband’s diagnosis, said she often plays the role of caregiver. “It’s me suggesting the residential program and then it’s me suggesting medication,” she said. “It has been such a fight each time and it’s very isolating because very few people have a behind-the-scenes look at what is going on.”

Belzberg’s perseverance and de la Rosa’s sometimes reluctant focus on his own well-being have helped. He is on a mood stabilizer and for four years has been in dialectical behavior therapy, which teaches people how to reframe their thoughts and behavior and helps them deal with distress. De la Rosa is now on track to get a master’s degree in sports psychology from the University of Western States, and he and Belzberg have built an online coaching business working with around 70 runners.

While research shows that most people with borderline personality disorder see an improvement in their symptoms with treatment over time, de la Rosa does not find comfort in that. He is frustrated by what he feels is a slow recovery and points out that a lot of people with the condition do not survive.

“For me, recovery has been like sitting in front of a TV screen on full blast a foot away,” de la Rosa said. “Years later, I’ve learned to put that TV upstairs, in a different room in my head, and turn the volume down a bit. I don’t think I’ll ever stop it from playing, but it’s not as loud as it used to be.”

By the time the Barkley Marathons came around this spring, de la Rosa felt ready to compete. When he decided his race was over a loop and half into the five 20-plus-mile loops it takes to complete the event, he was at peace with his decision and said he had learned an invaluable lesson.

“At Barkley I prepared for everything but my why,” said de la Rosa, who was coached by Belzberg. “Twenty percent of me was like, I want to finish this race because I still want to be relevant, so that people will still care about me and so I am not forgotten. But at 3 a.m. when it is pissing cold and my headlamp is going dim and the scenery is ugly and the food is terrible, what came to me is that I am worthy no matter what. My self-worth is not something I have to fight, fight, fight for.”

Suffering has always been a part of extreme sports. The sports and wellness journalist Alex Hutchinson said nobody enters an ultramarathon without expecting to test one’s physical limits. But Hutchinson, author of “Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance,” believes there is a shift away from the view that the point of running ultramarathons is to suffer.

“While the approach that ‘I am going to be an absolute glutton for punishment’ might get you through one ultramarathon, it is not likely to get you through 10,” he said.

When de la Rosa started running ultramarathons in the mid-2000s, his heroes were ultrarunners who glorified mental and physical suffering, seeing it as a sign of strength and commitment. De la Rosa believes he would have benefited if he’d found a coach or mentor who showed him that he could be successful and still be kind to himself from the beginning.

As a coach, de la Rosa champions the work of Steve Magness, whose book “Do Hard Things” focuses on how athletes can use positive self-talk when experiencing discomfort. De la Rosa now shows his clients that they need self-love to build resilience and mental toughness. He is trying to put those lessons in practice in his own training and life.

After struggling with depression and extreme exhaustion following Barkley, and with encouragement from Belzberg, de la Rosa has accepted that he cannot participate in multiday events and more extreme 100-mile races. For now, de la Rosa, who is a sponsored runner, will continue to work on his mental health and focus on single-day events.

Belzberg and de la Rosa, who have 21 rescue animals including cats, dogs, rabbits, guinea pigs, a rat, a 35-year-old pony and a pet crow, said that the moments of joy they have experienced running together far outweigh the tough days. One of their funniest memories is when de la Rosa paced Belzberg in one of her first 100-mile races and the couple hallucinated that they were seeing an aid station serving pancakes.

“I’ve always admired Nick’s determination,” said Belzberg, who is also a ballroom dancer and a writer. “I see it in aspects of his life such as his tackling B.P.D. He has such a strong willingness to try anything if it will help, and I think that’s a really commendable trait, and it’s there in his running, too. He has a remarkable capacity to blow up during races but still perseveres and even finishes very, very strong.”

If you are having thoughts of suicide, call or text 988 to reach the Suicide and Crisis Lifeline or go to SpeakingOfSuicide.com/resources for a list of additional resources.