Ed Sheeran on the emotional moments in his songwriting and the new documentary behind the scenes of his career

For Ed Sheeran, who’s had massive success as a singer, songwriter and guitarist, the art of writing a song is an intimate process with himself.

“When you're writing a song, you're laying yourself bare. And sometimes, saying things that might be quite embarrassing or coming out with ideas that might be quite embarrassing,” Sheeran told ABC News’ Chris Connelly.

He continued, “I don't know. Songwriting is a really weird thing. 'Cause I think it works as some sort of therapy for me, anyway. Like, I think I've stopped myself going mad by writing songs.”

The 27-year-old English artist is now the subject of “Songwriter,” a film from his cousin and video chronicler Murray Cummings that documents the creation of his 2017 album, “÷”

“I would always just have my back to a wall. And just observe what was happening. And just let it happen and film everything,” Cummings told “Nightline.”

While Sheeran has spent the summer playing to stadiums across North America, “Songwriter” puts the emphasis on his remarkable ability to craft intimate songs that have the power to enthrall young hearts. Sheeran says it’s less challenging to express “truth” in song than it is in real life.

“I think the best songs are the most truthful and honest songs. And it’s because it’s stuff you can’t actually say yourself,” said Sheeran.

For example, with his hit song “Perfect,” Sheeran said it was easier to express his feelings through the lyrics.

“It’s a nice thing to think that you’d say, ‘Oh, you look really good tonight.’ But to say for three minutes that -- and like every single bit of that -- might freak someone out a little bit. Whereas writing a song, it’s, like, I don’t know. It’s nice,” he said.

Even in bad times, Sheeran can channel his emotions through song.

“I can make myself happy by writing a song about my lowest points. If that makes sense. 'Cause you come out of it. And you're like, ‘Oh, well, that was a really bad day. But now I've got a really great song that will live on forever,’" explained Sheeran.

“He writes through his problems, definitely,” said Cummings. “And I think if you ever wanted to know anything about him, he's definitely written it."

The new documentary also displays Sheeran’s work ethic as a writer.

“I feel lazy when I'm not working. Like, when you finish a show and then you're on a bus, and you're like, ‘Well, I'm not doing anything for the next three hours,’ I'm either going to watch a DVD and drink a bottle of wine, or I can write a song. Or two or three,” he said, laughing.

Sheeran and collaborator Benny Blanco are shown in the film as they co-wrote “Love Yourself,” which eventually became a career-transforming hit for Justin Bieber.

“You kind of black out and then all of a sudden there's a song there and you're like, ‘What did we do for the last few hours?’ It usually happens at the most inconvenient time, like when we're on a bus traveling to another city,” Blanco told “Nightline.”

“Ed asked us both to leave the room. He was just like, ‘I'm just going to write some lyrics,’” said Cummings. “So, that's when I come back on the bus. And he's got the laptop open. And he's singing me what he wrote. That was really a moment where I got to see him build a song, like, from start to finish.”

Though it was a hit, Sheeran said the song wasn’t for him.

“I never wanted to perform that song ever,” he said. “It just didn’t fit. It didn’t fit anything. I didn’t want to sing that song.”

“Supermarket Flowers,” one of Sheeran’s more emotional songs hit close to home for the singer, who wrote it in the wake of his grandmother’s death.

“I never know how to, one, get grief out other than writing songs. But, two, that was my mum's mum who passed away. And I didn't know how to handle the situation with her,” he said. “So, my thing was writing a song.”

Sheeran described what it’s like being able to share his gift with the world.

“It’s a powerful thing, man. You know, songs might go off and make someone famous, or make the world care about them as a celebrity or something like that,” he said. “But when it’s that intimate moment when it's just you and your mum, and you're playing her a song about that, it's like all that stuff doesn't matter.”

He continued, “I would still do that. If no one knew my music, I would’ve still written a song and done the same thing.”

When he’s completed a song and performs it to thousands of people who sing it back to him, Sheeran said the song “goes full circle.”

“'Cause it's yours for so long, and you own it. And then, as soon as it goes out to the public, you no longer own it. And it's owned by Shelley, who had her first kiss when she was 15. Or, like, Derek has used it as his first dance or whatever,” Sheeran said. “It's now their song. And I think that's the final process. When the song comes out, you lose ownership of it. But the ownership is taken by people that really connect to the song and love it. And it becomes moments in people's lives, rather than your song.”

Watch the full interview on ABC News' "Nightline" TONIGHT at 12:35 a.m. ET.