Arvada, Colorado (CNN) Frank DeAngelis stood in his home office, his hair graying over his ears, and pointed to each frame on the wall, telling the story behind the mementos he's collected over the last two decades.

There are letters from President Bill Clinton, another from President Barack Obama and one from Vice President Joe Biden. There's a photograph of Frank with Clinton, another of him with Hillary Clinton, and one of him beside Celine Dion.

The torch holder he carried for the 2002 Winter Olympics in Salt Lake City is mounted near a matching newspaper clipping. And there's an autographed photo of baseball great Derek Jeter, wishing Frank well in the run-up to his retirement in 2014 as the principal of Columbine High School.

"It's a little bit of history," Frank said.

Even still, 20 years after two students stormed the Colorado campus, killing 12 students and one teacher, Frank DeAngelis is inextricably linked to Columbine -- and to the event that has come to mark a horrific and ongoing chapter in America's saga of school massacres.

At first, he was bound by a promise to stay at the helm until every student who'd been at the school that unimaginable morning had graduated. Then, he expanded that vow, remaining until every local child who'd been in class that day, down to preschoolers, had earned a diploma.

Framed memorabilia are displayed on Frank DeAngelis' office walls at his home.

Since stepping away from the principal's office, he has continued his commitment to collective recovery -- and expanded his flock far beyond Columbine High School.

Five years after retiring, the 64-year-old is as busy as ever, traveling the country to shepherd principals and communities that have fallen victim to the scourge of school shootings. It's the latest iteration of an evolving role, however unwelcome, he has pioneered since April 20, 1999.

"Columbine offers hope," Frank told CNN. "And that's what I hope, 20 years later, that we're doing, that we're reaching out to other people -- the Parklands, the Santa Fes, the Sandy Hooks, the Virginia Techs."

"I feel I was chosen to do that."

But he's also given so much of himself to Columbine, several people close to him said. And with the 20th anniversary of the shooting and the publication of a new memoir, "They Call Me 'Mr. De,'" Frank's wife, Diane DeAngelis, hopes he soon considers slowing down.

"It always comes to a head right before the anniversary," she said. "And I just hope that with the 20th, that maybe this is the last anniversary that is as big as it is and that we can move on a bit."

A devoted educator faces the unthinkable

When Frank was 13, he got a job in a pizzeria. In high school, he delivered newspapers. Frank's parents taught hard work and dedication, and when he got sick, he hardly ever missed work.

Diane, who dated Frank in high school, said he was nice but very serious. He didn't have a sense of humor. The couple spent all their time together, and while still in high school, Frank gave her a promise ring and said he wanted to get married. Diane didn't want that, she said, so they broke up.

"I had no spontaneity ... I was so serious," Frank admitted. "I was 15 or 16 going on 30, and I had to plan my whole life out."

Even so, Frank was unsure what he wanted to study in college, his brother said. But they had both played sports growing up, so when Frank told his brother he'd become an educator, Anthony DeAngelis assumed it was for the sake of athletics.

"I thought, 'He's probably going to be pretty good at this,'" Anthony said.

As with all things, Frank dove in deep. Early in his career, Frank's principal once forced him to fork over his keys to the school for a weekend. "He said, 'I do not want to see you around this school. Frank, you need to get away,'" he remembered.

Lights streak as a truck drives by Columbine High School in Littleton, Colorado, in this long-exposure photograph.

Frank displayed that same commitment to each of his students and the baseball players he coached, said Tom Tonelli, one of Frank's former pupils and a Columbine High School graduate who went on to teach at the school.

"It was always: Be a good student, be a good athlete, but above all else, be a good person," said Tonelli of Frank's expectations.

Still today, when Frank's brother hands over his credit card at restaurants, servers often ask if he's related to Frank, Anthony said. A waitress last year told him Frank had been her principal.

"And she goes, 'You could talk to any of my friends. What we appreciated was how he treated us,'" Anthony recalled.

That sentiment holds whether before or after the shooting, said Tonelli, who was on staff at Columbine the day gunfire erupted.

"Do I think the shooting transformed him? Absolutely," the teacher said. "But to say somehow he became a totally different type of person, I don't think so. The character he exhibited in the wake of the tragedy is just a reflection of who he was before it happened."

When 'the world didn't believe in us,' he did

Columbine High School serves a middle- and upper-middle-class community in Littleton, Colorado, where the mountains in the west rise into a wide open sky. Before the massacre, it was an "ideal" community, Frank said, with a lot of parental support and where he "could count on my two hands the number of fistfights we had in 20 years."

After the shooting, Frank "felt this enormous burden to go rebuild that community," he said. That's when he made the promise to stay at Columbine until the Class of 2002 had graduated. Other staff members made the same commitment, he said.

But in 2001, Frank felt he hadn't accomplished what he'd set out to do.

The Columbine Memorial in Littleton was dedicated and opened to the public on September 21, 2007.

"There were so many people deeply impacted, even the kids in elementary school," he said. "So, I made a promise that I wanted to be there until that last class graduated, which would be 2012."

Two years after that, he finally left.

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Frank's promise to stay gave him "so much credibility in the community," Tonelli said. The faculty and staff, along with the students and the whole community, looked to him as a leader, as someone who was "persevering for a cause greater than himself."

The perception stuck, even in the face of criticism that the school's administrators and faculty had fostered a student culture "where something like this could happen," Tonelli said, referring to the shooting and calling the claim "unjust."

The notion "that there were certain segments of the population we didn't care about was so untrue," the teacher said.

Through it all, Frank's "leadership meant everything," he said. "He was the biggest believer in our kids and in their teachers and in our community at a time when we felt like the rest of the world didn't believe in us anymore."

A leader battles darkness at home

But as he worked to help Columbine recover, Frank was also an ordinary survivor. At home, his heroic veneer vanished, giving way to the reality of post-traumatic stress disorder.

"I tried to do everything to protect what I call the Columbine family," Frank recalled. "But when I would come home, I just wanted to be left alone."

He didn't want to talk with his first wife and two stepchildren about what happened; they just didn't understand the aftermath, he said.

"It cost me my marriage," he said. "My wife was saying, 'You're not the same person I married. You've changed.' And I did. I felt so much guilt."

Religious icons are displayed in Frank DeAngelis' home office.

His trauma manifested in other ways, too. Months after the shooting, Frank and his brother went to a Colorado Rockies game. When fireworks lit up the sky, Anthony said, "My brother nearly took cover." Later, Frank told Anthony the celebratory display took him right back to the attack.

More shell shock set in when Frank returned to Columbine the summer after the shooting to prepare for the new academic year. Bangs and rumbles echoed as construction crews repaired damage to the building.

"I would have to go back to my office," he said, "and I would cry."

Hope thrives in 'tough love'

Ahead of the massacre's third anniversary, as he was pushed by divorce proceedings to the edge of emotional and financial "ruin," Frank began pecking away at the mountain of unopened letters he'd gotten in its wake. Among the first he picked up was from his high school sweetheart, Diane.

They began talking regularly by phone, often late into the night, but agreed not to see each other until Frank's marriage was dissolved.

"There was still a spark," Diane said, and she could tell Frank had grown up. "I could see that he had a sense of humor," she laughed, but also that his core traits hadn't changed. "Some of the good things that brought us together were there from the beginning."

But as their relationship developed, Frank continued to wrestle with his trauma. As with many Columbine survivors, it always got harder in the advent of April, a month in which Frank has gotten into six car wrecks and when his attention always jerks back to the terror.

Diane and Frank DeAngelis were high school sweethearts who lost touch, then married in 2013.

He leaned on counseling and his Catholic faith, but he was living alone in a nearly vacant house, with only a few pictures and a single bed left after most everything else was sold off.

"Twenty years of my life was in shambles," he said. "I was struggling," and he eventually started to drink.

Diane, whose father was a recovered alcoholic, quickly caught on. Frank started hanging up the phone around 4 in the afternoon, she said, and telling her they would talk the next day.

"Immediately, I knew," she said. "I thought, I don't know if I'm going to have to end this, because I can't go down that path again."

Diane's father died that April; Frank attended the visitation, and they began seeing each other. Soon, Diane caught him drinking. "I can't do this," she told him.

"It was justifiable," Frank said, looking back. "That was what I needed, that tough love ... I was so fortunate she came back into my life. And I didn't want to do anything to jeopardize that. It was a wake-up call."

Leading the 'Club Nobody Wanted to Join'

When he talks to others who have lived through school shootings, Frank mentions the risk of using alcohol or drugs to cope, and he emphasizes the importance of finding positive sources of support.

It's just one of many pieces of advice he gives to members of what he calls in his book, "A Club Nobody Wanted to Join."

Columbine wasn't the first school shooting, and it obviously wasn't the last. But every time another mass murder happens at a school, Frank said, his phone begins to ring with calls from reporters seeking insight from one of the nation's most seasoned campus attack veterans.

"Not that I'm an expert," he said, "but I lived through it."

He was called on as recently as this week to address the news media when a Florida teenager -- who authorities said was "infatuated" with the Columbine massacre -- traveled to Colorado and bought a shotgun, prompting the shutdown of Denver-area schools, including Columbine.

Quotes are inscribed at the Columbine Memorial.

Getting that call carries a lot of weight, said Andy Fetchik, the former principal of Chardon High School in Ohio, where three students were killed in a shooting in February 2012.

The first thing Frank said was, "We are now members of the same fraternity that neither one of us pledged," Fetchik told CNN. "And the second thing he asked me to do was to write down his cell phone number."

Months later, as Fetchik prepared to start the next school year, he gave Frank a call, he said.

"There was a peace of mind in speaking with someone that went through it," Fetchik said, noting that Frank validated the steps he'd taken to help his Ohio school community heal.

Several years later, Frank visited Chardon High School to talk with faculty members about the recovery process.

"One of the things I struggled with in the recovery was addressing the needs of staff. We didn't always know what they needed," Fetchik said. "Frank was that voice of somebody who's been there, who said, 'Where you're at is OK. Mental health recovery is not something you could control. There's no calendar.'"

'Columbine is not going to define me'

Today, Frank and Fetchik are members of the Principal Recovery Network , a new group of 17 current and former school administrators who have lived through school shootings and their aftermath. Unlike activists who have sought to change gun laws following campus attacks, these officials simply aim to offer themselves and their combined experience as a resource.

It falls in line with the work Frank has undertaken since he retired. Last year, he gave about 50 presentations in the United States and Canada about the recovery process. He also serves on the boards of school safety and other organizations, he said, knowing his name and connection to Columbine carry weight.

But he's tired, Diane said, and she's made it clear she hopes he slows down after the 20th anniversary of the event that has served as the pivot point for his life's work.

"He's doing a lot of good out there, and he has a lot to bring to the table," she told CNN. "But I worry about his health, because it hasn't been great. I see it in his face, how exhausted he is."

Frank DeAngelis stands outside his home.

For a man who's been working since he was a kid, "I can't imagine myself being completely retired," Frank said. And he knows he'll always want to help suffering communities. But he admits he needs to lighten his load.

"I'm looking at the 20-year remembrance as, I need to reevaluate," he said. "I need to be able to give myself permission to relax. I need to give myself permission."

When he retired, Frank said, Diane told him she worried he would fall into a depression because he would no longer be associated with Columbine. Around that time, he began worrying about his own health and suffered with anxiety. But the doctor told him he was fine.

Then, he visited another expert who pinpointed the problem. "You have been a part of Columbine for 35 years," Frank's therapist told him, he recalled. "And you feel that Columbine is Frank DeAngelis."

That perspective set the stage for a new outlook, Frank said. It's one he says he wants to embrace, though it may require as much determination as any hurdle he's conquered yet.

"He made me realize that Columbine is not going to define me. And that helped a lot," the former principal said. "I've just got to get it in my mind that it's OK."