After 30 years directing documentaries, Jennifer Fox was a specialist in the truth. Yet, she had been lying to herself since her 40-year-old track coach took her virginity when she was 13. An older, powerful adult man and a pubescent girl is textbook child abuse, but for much of Fox’s life, she explains, she waved off their evenings together as “my first relationship”.

The teenage Jennifer swore that Coach Bill (a pseudonym, as he is still alive) thought she was his secret love. He had flattered the lonely kid into considering herself his grownup equal. The word “victim” never crossed her mind. Instead, she was so confident their connection was special – unique, even – that she wrote a school essay about Bill and his accomplice, Mrs G, the horse-riding instructor who introduced them, rhapsodising about the pair as: “two very special people who I’ve come to love dearly ... I’m lucky enough to be able to share in their love.” Her teacher handed the paper back with the note: “Since you’re so well adjusted, it can’t be true.”

“It was the 70s,” says Fox, who is speaking from an airport, where she is in transit between screenings of her phenomenal new film, The Tale, framed around the rediscovery in her 40s of that class assignment. “If I had handed that essay in today, there would be flags all over it. Nobody was talking about sexual abuse and nobody was looking for it, certainly not in the affluent Jewish suburbs.”

The Tale, one of the breakout hits of this year’s Sundance, stars Laura Dern as a filmmaker named Jennifer Fox who flees her fiance Martin (played by the rapper Common) to head home and piece together her past. In flashbacks, we see a young Jenny hero-worship former Olympian Bill (Jason Ritter) and his imposing, and married, girlfriend Mrs G (Elizabeth Debicki). In the present, Dern tracks down an elderly Bill (John Heard) and Mrs G (Frances Conroy), and interviews her mother (Ellen Burstyn) and childhood friends. The documentarian turns her cool gaze on her own narrative: how did she hide the facts from herself?

That is a question a lot of people are wondering as #MeToo dredges up their painful pasts. The day before our talk, the Academy of Motion Picture Arts and Sciences expelled Roman Polanski and Bill Cosby – “Yes! Finally! Wow!” says Fox – but the conversation continues. At Sundance, Fox was startled to have “many, many men” tell her their own traumas. “The statistics may be way off,” she muses. Fox began to cross-examine her own history while conducting interviews for her 2006 film Flying: Confessions of a Free Woman. Other women’s tragedies sounded uncomfortably close to her young “romance”. “It just blew my mind that I was hearing stories that sounded just like my precious story that I had guarded as my identity, except they were calling it abuse,” says Fox. “It was not that I suddenly remembered more, because I always remembered everything. It was like a light went on on a part of the room that I had kept slightly dim.”

The Tale investigates the shadows where the brain – or in Fox’s case, the ego – hides unhappy ideas in self-preservation. The film feels like walking into a dark room with a torch and squinting to make sense of the shapes. It has changed a lot since her first draft, a straightforward chronicle of her relationship with Bill that, says Fox, just repeated the point: “Sexual abuse is terrible, sexual abuse is terrible, sexual abuse is terrible – definitely not the film I wanted to make. I wanted to tell a story about how a 13-year-old constructed the identity of this person that I became,” a tough, independent woman who leaps into dangerous situations and rejects marriage as bourgeois without questioning who implanted that idea in her head.

Fox calls The Tale “fictional memoir”. The character’s name, and story, is hers, a choice that protective people tried to talk her out of. “If I didn’t leave my name on it, this film would be too open to attack by people who’d say this can’t be true,” counters Fox. “The project needed me to stand up and vouch for its authenticity.”

One of the ironies of being a documentary film-maker is you are aware that it is almost impossible to swear you are telling the whole truth and nothing but. “There are multiple true narratives running at all times,” says Fox. “I made myself a hero out of an event where I was clearly not that, and there are also people who make themselves victims and they destroy themselves, as well.”

The facts are real; the dialogue is condensed from memories and research. Scenes shift underneath us as her life comes into focus. Clothes change, line readings shift, and the weather changes from snowy winter to autumn. Fox isn’t presenting the truth as much as pursuing it. As the opening title card announces: “The story you’re about to see is true – as far as I know.”

The Tale’s initial flashbacks star a teen actor named Jessica Sarah Flaum – the adult Jennifer’s imagination of who she had been. When Flaum stares into the camera and beams: “I always wanted to have a story to tell, but nothing ever happened to me before,” she sounds sophisticated and convincing. “At 13, you’re just yearning to live, and you see yourself as being much more capable and mature than you really are.” A few beats later, Dern pages through a scrapbook and realises that at 13, she had actually looked more childlike. That monologue rewinds and plays again with an even younger actress, the terrific Isabelle Nélisse, and this time when she says the line, we hear Jenny’s nervous bravado.

Nélisse and Dern grapple for control of The Tale, the naive girl finally glaring at the lens: “You want me to be some pathetic victim. Well, you know what, I’m not.” You respect young Jennifer’s strength – and then you realise how that strength has mutated into an adult who pushes people away. Fox protected Nélisse from the extreme scenes. She shot the girl’s sex scenes with a full-grown body double, and filmed her facial closeups standing against a vertical bed goofing around to non-sexual cues such as: “Act like a bee is stinging you.”

Still, these scenes do sting. They happened to Fox and countless other children. After decades of dodging the truth, she can now stare directly at her past. “Suffering is something you have to learn to figure out in your life; it’s nothing to be afraid of,” says Fox. “What’s really valuable is that, for the first time, people can see that, wow, this is really confusing for everyone. That thing which the media would like to paint in such black-and-white terms is very complex and nuanced. Feeling loved and loving and feeling special, children struggle with that while they’re being taken advantage of. That is what childhood sexual abuse is. So I guess I’m not so unique at all.”

• The Tale screens in the US on HBO on 26 May and premieres at Sundance London on 31 May

• This article was amended on 24 May 2018 to correct a picture caption.