After a three-year hiatus, the legendary Glastonbury festival returns to the U.K. this Friday with Billie Eilish as its youngest-ever headliner. The Grammy winner admits she's feeling the pressure but is trying not to "overthink it."

She tells Apple Music 1's Matt Wilkinson, "It's a serious once in a lifetime opportunity to be asked to go to and to do."

"I'm constantly feeling like I'm undeserving of everything, and I think that's a good thing... But first of all, it makes you doubt everybody because you're like, 'Why would you choose me?'" Billie added, noting that women aren't asked to headline as often as their male counterparts -- which makes the stakes even higher.

"If you overthink it, you might bomb. So I try to keep myself as composed as possible," the "Bad Guy" singer admitted. "I mean, there's always a hesitation just because you have to figure out timing wise like if everything works out."

And while Billie is "very geeked, very excited" about her upcoming Pyramid Stage performance, she also dove into what her "pre-show" ritual looks like.

The Oscar winner revealed she arrives "two hours" before the show and will "work out for a while" before getting ready. "I do my own makeup and then I do my own hair and I have to do it a very specific way. And then I have to find an outfit and then I have to make sure the outfit fits right," said Billie. After putting on jewelry, which she jokingly says takes "three hours," she tapes up her ankles and shins.

"And then I warm up my voice. And then I do more hair. And then I do another warm up. And then I go on stage," she revealed.

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