After falling from a horse in 1995 and becoming paralyzed from the neck down, Superman Relatives of actor Christopher Reeve say he at least hasn’t started wallowing in self-pity.

According to the new documentary Super/Man: The Christopher Reeve Story (in select theaters September 21 and 25), directors Ian Bonôte and Peter Ettedgui say the actor did his best to face each day with a positive attitude, despite the challenges he faced all day, every day.

“He allowed himself 15 to 20 minutes of compassion a day,” director Ettedgui tells PEOPLE of what he learned about Reeve while making the film.

“And that was it. Otherwise it was like there wasn’t a f—ing minute in the day to waste. He’d think, ‘I’m sad,’ but then you could almost see him calculating, ‘That’s it. Now it’s time to move on.'”

The directors and Reeves’ children are grateful to the actor’s wife Dana for helping him move forward after his spinal cord injury.

“My mom was probably the most extraordinary person who ever walked the earth,” Will, 32, says of Dana. “And it’s a testament to her unique spirit that she lived every day choosing hope. There was a relentless positivity that wasn’t manufactured, it was just who she was. And it helped move our family forward.”

He adds: “As a young woman, she was forced into these unimaginable circumstances, and my parents often joked, ‘We’re really testing that ‘in sickness and in health’ part of our vows.’”

Despite maintaining a positive attitude (and being committed to raising money for spinal cord injury research and making changes for the paralyzed community through the Christopher & Dana Reeve Foundation), the documentary doesn’t overlook how difficult life was for the actor afterward.

In the film, which uses both home movies and footage from Matthew’s previous documentaries about his father, Reeve can be heard talking about how he wanted to die in the days after he was told he would never walk again.

“I said, ‘Maybe we should let me go,'” Reeve said in archive footage. He said his wife responded, “I’m only going to say this once: I’m going to support whatever you want to do, because this is your life and your decision. But I want you to know that I’m going to be there for you for the long haul, no matter what.”

He said she then said the words that saved his life: “You’re still you. And I love you.”

Reeve lived for another 10 years after the accident, dying of heart failure in 2004 at the age of 52. In a cruel twist, Dana, a non-smoker, would be diagnosed with stage 4 lung cancer 17 months later. The film also explores how Will, who was orphaned at 13, coped after losing his parents at such a young age.

“Grief is permanent,” Will says. But he’s proud of the work he and his siblings continue to do with the foundation, and he’s thrilled to see his parents shine through Matthew and Alexandra, now parents themselves. He adds, “I think the most important part of the grieving process is healing, and you can find healing by honoring people you’ve lost in the way we live our lives.”

What does he think is the most important lesson Christopher and Dana have learned from the way they have lived their lives since the fall?

Will says, “We learned how important family bonds and love are for a fulfilling and meaningful life.”

For tickets to see Super/Man: The Christopher Reeve Storyvisit Fathom Events.

Want to read more about Christopher Reeve’s extraordinary life? Pick up the latest issue of PEOPLE, in stores Friday.