In Dancer, Steven Cantor’s new film about Sergei Polunin, there’s heartbreaking footage of the dancer when he was eight years old. Wide-eyed, gappy-toothed and lit by an irrepressible grin, little Sergei spins, tumbles and balances with a grace astonishing in one so young. As Polunin’s grandmother comments, “He used to dance with his heart. He transported himself right into the music.”

Nearly two decades later, as Cantor’s camera follows Polunin backstage after a show, the life has apparently been drained from him. Blank-faced and hunched, he mutters: “Every day I hope I will be injured, then I won’t have the option to dance any more.” A bleaker, darker fable than The Red Shoes, Dancer tells the story of how talent can turn from a blessing to a curse.

Something of that story became public in 2012 when Polunin, as one of the Royal Ballet’s most heavily promoted young principals, suddenly announced that he was quitting the company. Amid stories of cocaine use and his own gnomic tweets about “living fast and dying young”, the 22-year-old claimed that he’d become stifled by ballet, that “the artist inside [him] had died” and that he had to move on.

In fact Polunin moved on to Russia, where he joined the Stanislavsky Ballet in Moscow. Closely mentored by his new director, Igor Zelenksy, Polunin initially seemed to have made a fresh start – but news began to filter back that he hadn’t settled, that he was skipping rehearsals and missing shows and was again talking of leaving ballet, this time for a career in Hollywood.

It was hard get an accurate picture of what was going on. Polunin had become a magnet for journalists, and during interviews he tended to blurt out whatever was passing through his mind. With his words often taken out of context, he was frequently presented as an ungracious, lazy, confused young man with delusions of celebrity. The larger story, of Polunin’s difficult family background and unresolved demons, was less widely known. It’s this story that Cantor tries to tell in his humane and sympathetic documentary.

Cantor is a director, not a dance expert, and perhaps it’s not surprising that at times he oversimplifies his material in the service of his plot. His use of heavy rock guitar and staccato pacing to colour the scenes of Polunin’s early rebellion make the dancer seem wilder than he really was (drugs and tattoos are far from unknown in the ballet world). There is no attempt to place the dancer’s gifts or the trajectory of his career within the wider context of ballet.

Cantor’s focus, fairly enough, is all on Polunin and on the troubled and complex nature of his talent. He has accessed some marvellous film archives, which give revelatory proof that Polunin was a natural prodigy. We see him at the age of 11, his skinny limbs already shaped by a beautiful line and precocious control; we see him as a teenage student at the Royal Ballet School, leaving his classmates behind as he powers through a complex bravura variation.

But raw talent, however astonishing, may not enough be enough to nourish a career, and Cantor vividly sketches the narrative of how Polunin went from infant prodigy to angry rebel. He was born in the drably impoverished town of Kherson, south Ukraine, but while he recalls his early childhood as happy, his mother, Galina, had her eyes set on wider horizons and enrolled him first into gymnastic classes and then into ballet. As the extent of his talent became clear, she was determined to make him a star.

Everything was sacrificed to that end. When Polunin was given a place at the ballet school in Kiev his father, Vladimir, went to work in Portugal, and his grandmother to Greece, in order to pay the fees, while Galina gave up her own life in Kherson to go to Kiev with her son. At 13, Polunin won a place at the Royal Ballet School, moving to London, where he knew no one. He spoke not a word of English. For two years he apparently flourished, but his determination to be top of his class was driven not only by his own desire to do well but by the belief that, as a successful dancer, he would be able to provide for his family and bring them back together.

The family, however, did not survive its enforced periods of separation and when Galina and Vladimir divorced two years later, something seemed to have broken in the 15-year-old Sergei. Although he continued to make exceptional progress, Polunin recalls that he was very angry, very unhappy inside. He danced through his demons, but by his early 20s he had achieved most of his professional goals, and lost all his motivation. The joy had gone from his work, his family were no longer around to benefit from his success – and in any case he now wanted little to do with them, especially his mother, whom he blamed for having forced him into a career he’d never chosen.

Some of the most revealing interviews in Cantor’s film are with the family. Polunin’s father seems genuinely regretful and shocked by the revelations of his son’s unhappiness, saying he had always been so worried about being the bread-winner that he’d failed to hold his family together.

Galina remains adamant that everything she’d done was correct. There’s a bleak, awkward conversation during which Polunin tries to explain to his mother how coerced and miserable he had felt, but she simply reiterates that he had to take responsibility for all the sacrifices the family had made for him. I hope Cantor was fair to Galina in the editing because she doesn’t come out of the film particularly well: there’s a marked contrast between the wariness Polunin shows towards her and the emotion with which he embraces his first ballet teacher when he goes back to Kherson and recalls the time when ballet was still an innocent, joyful thing for him.

I’m guessing that Cantor considered ending his film with the video for Hozier’s Take Me to Church, which Polunin made in 2015 with the artist David LaChapelle. In one of the final interviews to camera, Polunin says that the video was to be his formal farewell to ballet, that he would give up dancing and get himself a “normal life”. But events moved on after Take Me to Church went viral. Polunin became less adamant about retiring and Cantor had to be content with a less conclusive ending.

Cantor didn’t prolong the filming long enough to catch Polunin as he met and fell in love with the Russian ballerina Natalia Osipova, a relationship that Polunin claims has transformed his life. The dancer today is very different from the edgy, confused subject of Cantor’s documentary; he says that with Osipova he has learned to love ballet again and has the confidence to carve out the career that he wants. He will perform and present repertory that he believes in – this March he is premiering Project Polunin, an independent programme of new and vintage ballets, which includes some of his own choreographic input. He is also planning to set up a management agency that will enable dancers to work independently of a home company and theatre, as actors and singers tend to do.

It’s too early to forecast the success or otherwise of Polunin’s plans. In recessionary times, it’s harder than ever to fund independent dance projects (Will Tuckett’s Nutcracker recently fell victim to financial troubles). And it takes guts, good taste and rigour for a dancer to steer their own career. From the evidence of Cantor’s film, it’s impossible to know how well equipped Polunin will be for his new career; how easy it will be for him to leave his restless demons behind.

There is one other unresolved issue that arises from Dancer: what, if anything, could have been done to avert those early crises? When he first walked out of the Royal, he claimed it was the company’s fault and that there was an English “mafia” in the company intent on pushing out Russian dancers (in “Russian”, he presumably included himself).

These days, though, Polunin’s tone is far less accusatory. When I interviewed him (with Osipova) last year, he was touchingly eager to correct any idea that he had not been supported in London: “The Royal Ballet School looked after me very well, they were like my family, and the company gave me everything.” But he did want to explain how alone he had felt back then, and how unable he was to cope with his alienation and anger.

“I was unhappy [at school] and I didn’t know how to express it,” he said. “At home if you were angry you had a fight with someone, but at the school no one ever fought – you would have been thrown out. In the company I began to feel lost. I wanted to do other things, like a musical or a movie, but I was afraid of messing up. I had lived in London for 13 years – it was my home. but I wasn’t a citizen. If the director was angry with me and threw me out, where would I go? When I walked out, I think I was trying to make the worst thing happen to me, the thing I was most scared of, so that I wouldn’t be frightened any more.”

The process of self-reflection that Polunin clearly underwent in the filming of Dancer, along with the confidence he has acquired with Osipova, has brought the dancer an emotional clarity that was beyond his reach a few years ago.

Student dancers and young professionals are a rarefied breed; they’ve been hot-housed to an early maturity; many have left behind their families and homes and all have developed a high degree of competitiveness and self-criticism. As with classical musicians and elite sportspeople, it’s difficult for them to admit any kind of weakness, even to themselves. If the busy teachers and staff at the Royal failed to observe that Polunin was struggling, so did most of his peers.

But this culture of stoicism makes it imperative for the dance industry to develop better systems for dealing with issues of anxiety, burnout and stress. The Royal provides excellent medical facilities to prevent and treat its dancers’ injuries. But just as society as a whole is waking up to the scope of mental health issues among the general population, so dance needs to get better at identifying the problems that can affect its performers. There are other outstanding talents that have been squandered or spoiled by a lack of intelligent nurturing – ballerinas such as Gelsey Kirkland and Bryony Brind are prime examples of those who struggled with demons and doubts. Dancers need to know that it’s OK to ask for help, and management need to create a culture where vulnerability isn’t equated with failure.

• Dancer is released in cinemas on 10 March 2017. The premiere with a live performance from Sergei Polunin is on 2 March at the London Palladium and will be broadcast into cinemas nationwide. Project Polunin is at Sadler’s Wells, London, 14-18 March.