Teen sensation Billie Eilish revealed she was "clinical depressed" during the biggest year of her career.

The 17-year-old Canadian singer, by far the greatest success story of 2019, spoke at the inaugural Apple Music Awards yesterday where she got candid about a difficult time in her life.

Eilish, whose star began to rise in 2017 with her debut EP, Don't Smile at Me, released her debut studio album, When We All Fall Asleep, Where Do We Go? in March this year, which remarkably debuted atop the Billboard 200 and became the best-performing album of 2019 in the US, as well as reaching number one in the UK, Canada and Australia, reports News.com.au.

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She is also the youngest artist in history to be nominated for the "big four" Grammy Awards in 2020; Record of the Year, Album of the Year, Song of the Year and Best New Artist.

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Despite such rapid success, Eilish said she had been "pretty clinically depressed for a couple of years".

"I was probably the most depressed I've ever been … It's interesting to see interviews from back then because you would probably never really notice," she told Apple Music's Zane Lowe.

"I was not doing well, there were all these amazing things happening but still I wasn't there. "Even after the album came out I felt the same way. It took a couple of months to start to feel OK again. That wasn't because of my career, it was just because I wasn't in a great place.

Eilish, winner of New Artist of the Year and Favorite Artist - Alternative Rock at the 2019 American Music Awards. Photo / Getty Images
Eilish, winner of New Artist of the Year and Favorite Artist - Alternative Rock at the 2019 American Music Awards. Photo / Getty Images

"I remember last year Ariana Grande did a speech where she said last year was the best year of her career and the worst year of her life, and I understood it."

Eilish performed a dazzling acoustic show for Apple Music at the Steve Jobs Theatre at Apple Park in San Jose as she was honoured with their first Global Artist of the Year award.

In a better place now, Eilish said she has focused on changing her way of thinking.

"A month ago I decided to start living like I went to sleep at 12, a little fan of every artist I was a fan of and I woke up and this is my life. How would I feel then?"

Apple are clearly big fans of Eilish, the tech company reportedly signing $US 25 million ($AU 36.5 million), for the right to stream an intimate documentary about the star on its new streaming service, Apple TV+.

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It comes after an interview with Vanity Fair last month, where Eilish recalled an emotional moment in January when she received news of her chaotic schedule for the year.

"I had a meeting showing me the schedule for the whole year and I sat there and was totally silent and still and just tears streamed down my face," she said.

"I went to my garage and I just cried. It was just very overwhelming and I was in a really bad place."