“From the moment I woke up until I went to bed, it was all day screaming in my face, yelling at me, continuous torture,” Paris Hilton recalls in an exclusive interview with PEOPLE

Paris Hilton is opening up about a painful secret she’s kept private for over two decades.

In her new documentary, This Is Paris, premiering Sept. 14 on Hilton’s YouTube channel, the entrepreneur and reality star reveals for the first time the horrific abuse she says she endured as a teen while at a boarding school in Utah.

“I buried my truth for so long,” Hilton, 39, tells PEOPLE exclusively of the mental, emotional and physical pain she says she underwent while at Provo Canyon School in the late ’90s. “But I’m proud of the strong woman I’ve become. People might assume everything in my life came easy to me, but I want to show the world who I truly am.”

Years before she became a household name on The Simple Life in 2003, Hilton was a teen living in New York City’s famed Waldorf Astoria Hotel with her parents, Rick and Kathy Hilton, and younger siblings: Nicky, 36, Barron, 30, and Conrad, 26.

Paris Hilton Cries over Childhood Trauma in Documentary Trailer: ‘No One Really Knows Who I Am’
And along with the privileges of her big city life came big temptations.

“It was so easy to sneak out and go to clubs and parties,” recalls Hilton. “My parents were so strict that it made me want to rebel. They’d [punish me] by taking away my cell phone, taking away my credit card, but it didn’t work. I would still go out on my own.”

Eventually, says Hilton, Rick and Kathy were fed up and made the decision to send their then 17-year-old daughter to a series of boarding schools that claimed to focus on behavioral and mental development, the last of which was Provo Canyon School, where Hilton would stay for 11 months.

Almost immediately after she arrived, “I knew it was going to be worse than anywhere else,” says Hilton. The abuse she faced, she says, took place on a daily basis.

“It was supposed to be a school, but [classes] were not the focus at all,” says Hilton. “From the moment I woke up until I went to bed, it was all day screaming in my face, yelling at me, continuous torture.”

(When reached by PEOPLE for comment on the allegations, the school responded: “Originally opened in 1971, Provo Canyon School was sold by its previous ownership in August 2000. We therefore cannot comment on the operations or patient experience prior to this time.”)

Continues Hilton: “The staff would say terrible things. They were constantly making me feel bad about myself and bully me. I think it was their goal to break us down. And they were physically abusive, hitting and strangling us. They wanted to instill fear in the kids so we’d be too scared to disobey them.”

Four of Hilton’s former teen classmates also appear in the documentary, making similar allegations about Provo Canyon School, including that they were often force-fed medication and held down by restraints as punishment.

When one of her classmates told staff that Hilton had plans to run away (“you couldn’t trust anyone there,” she says), she says she was placed in solitary confinement. “They would use that as punishment, sometimes 20 hours a day.”

The fear of continued abuse began to take its toll on the once vibrant teen.

“I was having panic attacks and crying every single day,” says Hilton. “I was just so miserable. I felt like a prisoner and I hated life.”

Attempts to tell her parents about the conditions at school were fruitless. “I didn’t really get to speak to my family,” says Hilton, “maybe once every two or three months. We were cut off from the outside world. And when I tried to tell them once, I got in so much trouble I was scared to say it again. They would grab the phone or rip up letters I wrote telling me, ‘No one is going to believe you.’ And the staff would tell the parents that the kids were lying. So my parents had no idea what was going on.”

Finally, when she turned 18 in 1999, Hilton left the school and headed back to New York, but was terrified to speak a word of her experience — to anyone.

“I was so grateful to be out of there, I didn’t even want to bring it up again,” says Hilton. “It was just something I was ashamed of and I didn’t want to speak of it.”

But more than 20 years later, in filming the documentary and reflecting on her life thus far, Hilton says she was finally able to open up about the trauma of her past — in the hopes that she can finally move on.

“It feels like my nightmare is over,” she says. “And I’m going to watch the movie with my parents — I think it will be good for us, but emotional too. There are no more secrets.”

Hilton says she doesn’t have any plans currently to pursue legal justice; instead, she’s focused on raising awareness about other so-called behavior improvement schools that she says still employ as a practice the kind of physical and verbal abuse she endured for so long.

“I want these places shut down,” says Hilton. “I want them to be held accountable. And I want to be a voice for children and now adults everywhere who have had similar experiences. I want it to stop for good and I will do whatever I can to make it happen.”

This Is Paris premieres Sept. 14 on Hilton’s YouTube channel.

Look for more from Hilton in an upcoming issue of PEOPLE.