Paris Hilton still has ''nightmares'' about an ordeal in her childhood.

The 39-year-old heiress opens up about the mental abuse she fell victim to as a teenager at boarding school in her 'This Is Paris' documentary series and she admitted it's a subject she finds very difficult to talk about.

In a preview clip obtained by People, Paris said: ''I'm nervous. I'm shaking.

''It's hard to even eat, because my stomach is just like, turning.

''I don't know -- it's something that's very personal, and not something I like talking about.

''No one really knows who I am. Something happened in my childhood that I've never talked about with anyone. I still have nightmares about it.''

A synopsis for the documentary explained the 'Stars Are Blind' singer will confront her ''heartbreaking trauma''.

It reads: ''This Is Paris uncovers the hidden past of the international icon. As Paris confronts the heartbreaking trauma that forged who she is today, this deeply compelling portrait tells the real story of a teenage girl desperate to escape into a fantasy and sheds new light on the insta-fame culture that Paris helped to create.''

Paris previously admitted she had found making the film - which premieres in September - therapeutic.

She said: ''I became so close with the director, and she really asked me so many questions.

''I started thinking about my past and what I've been through and realised just how much it's affected my life and how I was holding on to so much trauma for so long.

''I think when people see this film, they're really going to see a different side to me than they've ever seen before.

''Because I honestly didn't even know who I was up until this year. I really learned a lot about myself through this film.''