Demi Lovato dives into the mental health challenges that come with fame at a young age in a new documentary − and in it she reveals something experts say is key to understanding trauma.

In "Child Star" on Hulu and Disney+, the 32-year-old singer said there are large parts of her time on set when she was a teenager that she doesn't remember.

At one point, Lovato tells her former "Camp Rock" co-star Alyson Stoner, 31, that she remembers filming most of the 2008 Disney Channel original movie, but not much of its 2010 sequel "Camp Rock 2: The Final Jam." Stoner described having a similar experience.

"Disassociation: It's a common thread between all of us," Lovato says.

Mental health experts say that both dissociation and disassociation − two terms often used interchangeably − are common for those who've been through trauma. In order to protect itself, the brain dissociates, or blocks out certain memories and experiences.

"It's a defense mechanism where you shut down in order to survive what you're going through," psychotherapist Stephanie Sarkis says. "It's your brain's way of protecting itself. And so, it's a way you are able to function with your trauma."

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What is dissociation?

Dissociation can look different for different people.

Some have blank spaces in their memory from when they dissociated. Some dissociate during a traumatic experience, while others dissociate while recalling a traumatic memory. Some imagine going to a dark space in their mind where they feel safe when they dissociate. In another form of dissociation, called derealization, people may experience time in slow motion, or their surroundings may not feel real.

Some people who dissociate may see themselves as an outsider to a traumatic event, as if they were watching themselves from outside their own bodies.

"You'll hear people talking about how they feel like they're in the room, but they're looking down, and so what's happening isn't happening to them. It's happening to someone else," says therapist Philip Lewis. "And then it can also happen later on when you're having a trauma response."

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In a conversation with Raven-Symoné, who acted as a child on "The Cosby Show" and "That's So Raven," in the documentary, Lovato reveals she can't recall working with Symoné on Disney Channel's "Sonny with a Chance," which Lovato starred in, until Symoné reminded her of it much later.

"It was part of my disassociation that I don't even remember so much of my show that I was on," Lovato says. "But I do remember how difficult I was to work with, because I was in so much pain, and I was hurting."

Symoné said she understood − and recalled noticing that Lovato didn't seem fully present when they worked together.

"I mean, you weren't the nicest person," Symoné said. "But being the type of person I am, in that I've been in the industry for as long as you, and I understand the glaze over the eyes, I didn't hold it against you. I just was like , 'Something's going on there.'"

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Clinical psychologist Kevin Chapman says the glazed look described by Symoné is typical for people dissociating.

"It's like they're physically present, but they're mentally not present," Chapman says. "You can see that they're actually checked out in some way."

It's possible for people who dissociate to remember traumatic memories they previously blocked out. One of the goals of trauma therapy is to gently guide someone into recalling details of a traumatic event they may not fully remember. By doing so, the aim is to allow the person to face their trauma so they can heal from it.

"That's one of the goals of trauma treatment is to first establish a rapport with a person, establish safety in the room, remind them of their autonomy, that they are in control of themselves, that they're not in a state of danger, where they're out of control, and then to help them start going through what happened," Lewis says. "It can be a very long process, but you start having them talk about what happened bit by bit."

More:A lot of people talk about 'complex trauma.' What does it mean?

What else has Demi Lovato said about child stardom, mental health?

Lovato got candid during a recent interview with The Hollywood Reporter about her highly publicized struggles with fame and her collaboration with Disney for a documentary on child stars.

"I think part of me always thought that if I made it in the industry that I would get the love from my birth dad that I didn’t have. And he was troubled, and I think I always chased success because I knew it would put me in his line of sight again and it would make him proud of me," Lovato told the publication in an article published in August.

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She continued: "But now that I’ve dealt with those daddy issues, I don’t need the industry as much as I once did, and I’m proud of myself for getting here."

Lovato's latest documentary project serves as a follow-up to her YouTube Originals docuseries "Demi Lovato: Dancing With the Devil," which followed the star's struggles and aftermath of a harrowing 2018 overdose.

Contributing: Jay Stahl