When Lisa Selby was 10 months old, her mother Helen dropped her off at the babysitter’s and never came back. In this frank, intimate and incredibly moving documentary, we watch Selby interviewing her mum, a heroin addict and alcoholic, dying of cancer. In her filthy basement flat in south London, cans of Special Brew on the table, Helen answers her daughter’s questions. Are you maternal? “No! I’m terrible! I’m not a responsible parent.”

As a child, Selby idolised her beautiful, glamorous absent mother. Twice a year, on birthdays and Christmas, cards arrived with Helen’s “autograph”; “she was better than Madonna to me,” says Selby. In a beautifully written voiceover she speaks with painful honesty about the feeling of longing she’s never been able to shake off. She wonders if she can be a mother herself, or if it would be better to break the cycle. “Mum: the word that doesn’t feel good. It’s not my word.”

This makes Blue Bag Life sound like a closet-clearing misery memoir. And the year that her mum dies is horrible for Selby; her partner Elliot also starts using heroin again after getting clean and is jailed for drug dealing. But the funny thing is, the film is not all woe, misfortune and tragedy. Selby’s childhood was damaged, but not loveless. After Helen walked out, she was raised by her wonderful dad (“mum and dad rolled together”) on a council estate in east London, and grew up to be an artist and university lecturer. Elliot comes out of the other side of his relapse, and the couple documented the journey through prison and recovery on the Instagram page bluebaglife.

Selby shares directing credits here with documentary maker Rebecca Lloyd-Evans and editor Alex Fry, who have sensitively assembled the film from videos Selby shot over the years – a lot of it on her iPhone, some of it extremely raw (“emotional hoarding” is her description of the piles of hard drives). There’s even a bit of humour. When Helen dies, Selby takes charge of the death admin. The council official writing the death certificate umms and ahhhs about what occupation to list for the deceased. He decides on “homemaker”. This cracks Selby up: “You should see her home!”

• Blue Bag Life is released on 7 April in UK and Irish cinemas, with an Australia release date to be confirmed.