Prince William lost his public speaking nerves thanks to his deteriorating eyesight.
The 37-year-old future king admitted he used to experience a "bit of anxiety" ahead of having to address a crowded room, but when he was unable to distinguish faces because of his poor vision, he felt more confident as he didn't feel under so much scrutiny.
He said: "You're like, 'This has to go right. I don't want to mess this up'. There's a lot of people watching and you can see certain people.
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"My eyesight started to sort of tail off a little bit as I got older, and I didn't used to wear contacts when I was working, so actually when I gave speeches I couldn't see anyone's face.
"It helps because it's just a bit of a blur of faces, and because you can't see anyone looking at you. I could see enough to read the paper and stuff like that, but I couldn't actually see the whole room.
"And actually that really helped with my anxiety.
"I didn't realise at the time but looking back I'm like that's what helped because I couldn't see everyone's eyes, you don't feel like the whole weight of the room is watching you."
Elsewhere in the documentary, titled Football, Prince William and our Mental Health, the royal - who has Prince George, 6, Princess Charlotte, 5, and Prince Louis, 2, with his wife, the Duchess of Cambridge - admitted becoming a father brought back the trauma of losing his mother, Princess Diana, in a car crash in 1997.
Speaking to Marvin Sordell, who quit football last year due to depression, he opened up after the former Watford striker admitted becoming a dad in 2017 brought back painful memories of growing up without his father.
William said: "Having children is the biggest life-changing moment, it really is.
"I think when you've been through something traumatic in life and that is, like you say, your dad not being around, my mother dying when I was younger, the emotions come back in leaps and bounds.
"Because it's a very different phase in life and there is no one really there to help you. I definitely found it at times very overwhelming."
• Football, Prince William and our Mental Health is airing now on the UK's BBC One channel