How Writing This Famous Song Freed Dolly Parton from Her 'Love-Hate Relationship' with Porter Wagoner

Dolly Parton knew her roots and was well-aware of how her career started. But she also knew when it was time to break free.

Parton is the latest guest on filmmaker Ken Burns' UNUM project. In a PEOPLE exclusive clip from his film Country Music, the 75-year-old speaks about the moment she decided to terminate her musical partnership and long-term working relationship with Porter Wagoner in 1974, after working together for seven years.

"I think Porter had a real hard time after other people started recording my songs. And I was writing and I was getting to be pretty popular," Parton says in the PEOPLE exclusive clip. "And it was his show. I wasn't trying to hog it. But I just kind of carved out a little, you know, place for myself."

"But it was a love-hate relationship. We fought like cats and dogs," she adds. "We were just both very passionate people. There was no way that I wasn't going to do what I was going to do. And no way I was going to not do what he thought I was going to do."

Parton and Wagoner started working together when she joined his The Porter Wagoner Show in 1967. They also recorded several albums together. He wanted Parton to continue on his show, while she wanted to go off and do her own thing.

"When I was trying to leave the show, I had told Porter I'd stay five years, it had been five, and it was six, and it was seven," she says in the video. "He was just having a real hard time because it was gonna mess up his show. We were very bound and tied together in so many emotional ways. And he just would not hear it."

Wagoner threatened to sue her. Parton ended up communicating with him the best way she could: she ended up writing a song.

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"I thought 'Do what you do best, just write a song.' So I wrote the song, took it back in the next day. And I said, 'Porter sit down, I got something I have to sing to you.' So I sang it. And he was sitting at his desk and he was crying," Parton recalls. "He said, 'It's the best thing you ever wrote. OK, you can go, but only if I can produce that record.' And he did and the rest is history!"

The song ended up being Parton's best-selling song: "I Will Always Love You," which was later recorded by the late Whitney Houston.

Wagoner would go on to sue Parton for $3 million. Parton would settle for $1 million and she'd pay him in installments over several years. They'd go on to forgive each other and recorded a reunion album in 1980.

"Decades later, when Whitney Houston re-recorded the song and made it the best selling single by a female artist of all time, Dolly retained her songwriter's rights, saying that she had made enough money from the release of Whitney's version to buy Graceland," Burns says in the clip. "For decades, women in the music industry have been forced to fight for control of their music and lives, often not so successfully. This story is not new. This is just Taylor's version."

Referring to Taylor Swift and the re-recording of her own music after the masters of her songs were acquired by Scooter Braun, Burns tells PEOPLE, "While Taylor's version may be the most recent occurrence of this story, it's certainly not the first."

"For decades, women in the music industry have been forced to fight for control of their music and lives, often against men exploiting their talent and fame for financial gain," Burns adds. "In the early 1970s, Dolly Parton fought a bitter legal fight over ownership of her music and career with a man who felt responsible for her success and thus entitled to a significant portion of her earnings."

UNUM, Burns' latest project, uses clips from his work to explore the intersections between history and current events.