Rufus Wainwright's new project is his most personal yet, an opera inspired by his dying mother, folk star Kate McGarrigle

• Rufus Wainwright

Not only was Kate a major inspiration behind Prima Donna, but her funeral reflected the importance of music – and specifically opera – in the life of both McGarrigle and her son.

"Opera has been my religion and saviour. It has saved my life, guiding me through some pretty tough junctures," Wainwright tells me when we meet at Sadler's Wells in London, where Prima Donna is being staged this week, following last year's premiere at the Manchester International Festival.

Surprisingly tall at six foot one, with that cartoon handsome face, Wainwright looks shattered. But his sense of humour is irrepressible. "It's been such a bittersweet time. Because on the one hand I accomplished my main goal, which was for my mother to see the opera in time. We had a wonderful experience doing that. But on the other hand she did eventually pass away. And the piece survived. Which is kind of amazing because, you know, with classical critics … it could have died as well.

"Yes, it is tinged with sadness now, but what opera isn't?", he says, roaring with laughter.

Set on Bastille Day 1970, Prima Donna is the story of a fading opera queen who mysteriously quit the stage at the height of her powers and is now on the brink of a comeback. New York's Metropolitan Opera originally commissioned Wainwright, but the collaboration broke down when he revealed that he wanted to write it in French and that he couldn't afford to wait until 2014, when the Met had its next free slot – his mother had just been diagnosed with liver cancer. He says: "I knew immediately that I would probably have to switch opera houses."

Wainwright's inspiration for Prima Donna – a two-hour opera for four singers and full orchestra – was seeing interviews with the late Maria Callas. But he also draws from Gloria Swanson in Sunset Boulevard and the cult 1981 French thriller, Diva. And of course, his mother's illness.

In the past he has described the dysfunctional Wainwright family as "the Addams Family of the block".

He had a stormy relationship with his father, Loudon Wainwright III. Loudon left Kate when Rufus was three and the children grew up with their mother in Montreal. One of Rufus's most memorable songs, Dinner at Eight, recounts a furious row which saw the two almost coming to blows. Sister Martha went one better with her single Bloody Mother F***ing Asshole.

Neither parent handled it well when Rufus told them he was gay and for years Rufus and Martha had a close but prickly relationship. But all that is past now, he tells me. Kate's illness changed everything. Not only are he and Martha (who has just given birth to her first child, Arcangelo) now very close, but Loudon was there when Kate was dying. Rufus says: "My dad was absolutely marvellous for this whole period. He really came to the fore and protected his children and reached out to his ex-wife and made a concerted effort to bury a lot of hatchets. As did my mother. It wasn't like they kissed and made up and went out to dinner, but they came together finally in a beautiful way."

The funeral was full of music. "There was an open wake," Janis Kelly says. "They had a piano and a friend of Rufus's played a Brahms piece in the style of Kate's playing. Then I sang the last aria from Prima Donna, which just felt right. Then next day I sang In the Bleak Midwinter as they brought the coffin in and there was extraordinary singing from Martha and Rufus. I found it very moving and inspirational. It wasn't morbid at all."

When Rufus first announced he was writing Prima Donna, the opera world was divided. Could a pop star really pull it off? But Rufus has worshipped Verdi since the age of 14 – Greek Song on his 2001 album Poses is taken from Weber's Der Freischtz – and his intense love of classical music was, he acknowledges wryly, a way of rebelling against the "banjo-playing macho folk music" of his parents. He and Martha would act out Tosca to escape from the well-documented complexities of Wainwright family life. It sustained him during the flaunting of his homosexuality that led to a sexual assault in Hyde Park at 14 (when he was staying in London with his father); through his descent into crystal meth addiction (he temporarily went blind, and was guided to rehab by Elton John); and when his mother was first diagnosed in 2006.

Opera became a shared passion with his mother. "She knew a lot about tenors but she'd never seen a whole opera. So that was a journey we took together," he says, close to tears. "The last three years of her life were very connected with opera. For her 60th birthday I bought 11 high-priced box seats at the Met for her and her friends. We went to see La Traviata (the story of the dying Violetta], and it was a gorgeous experience. But three months later she was diagnosed and now when I think about it, oh my God, the parallels were eerie. Then, in the middle of her illness, we went to the Wagner festival in Bayreuth. She had been in the hospital for a long time with a wound from surgery that wouldn't heal, and we went to see Parsifal, which is all about that. And then the last opera we saw was Orfeo at La Scala, about going into the underworld. Opera is definitely mystically inclined in my life in many ways."

Later, when he was writing Prima Donna, his mother was a major support. And now Rufus has new stability in his life: he is in a relationship of three years with German theatre producer Jrn Weisbrodt. He spends a lot of time in Berlin. But for the next two months, London is his base. He knows the city well – "I have an interesting relationship with London; I've had the best and worst of times here" – and says he will be bringing a guest list of the "fab folks" to the premiere: "That's one thing London is amazing at."

His ambition for Prima Donna is that it will draw in the Lady Gaga generation. "In opera they desperately need a young audience. Sorry to say it, but their crowd is dying off. But on the other side of the spectrum, the young pop-going populace is getting slightly shafted by corporate interests and technological flattening of live performance, so they're ready for each other."

At the opera's premiere, Rufus took a bow dressed as Verdi. Kelly says when that they were looking for her understudy there was a joke that Rufus could play her role himself. He's inclined to agree. "The title of my new album, All Days Are Nights: Songs for Lulu, represents the destructive, volatile, brooding female force that I believe everyone has, whether you are male or female."

The new album is inspired by Louise Brooks's character from the 1929 film Pandora's Box, but shot through with grief for Kate. Artist Douglas Gordon has made a film and his friend, Zaldy, Michael Jackson's costume designer, devised a 17ft-long gown for him to wear at live performances. It's suitably funereal, but actually Rufus had the idea several years ago. "My idea about music in general is that all creations are clairvoyant. It was me preparing for my mother's death. And to mourn it appropriately on stage where we spend most of our lives."

The highlight will be when he comes to sing his own version of the album's standout track — Les Feux d'artifice t'appellent. "The lyrics, 'The fireworks are calling you/descend into the streets', that is my mother saying, 'Life is short, go down and enjoy yourself because it's over in seconds'," he says, visibly moved. "As an artist you begin to prepare for things before you even know they're happening."

Prima Donna is at Sadler's Wells, London until 17 April, www.sadlerswells.com All Days Are Nights: Songs for Lulu is out now on Polydor. Rufus Wainwright plays Glasgow Royal Concert Hall tomorrow, and the Usher Hall, Edinburgh, on Friday.