In May 2016, with the launch of their Heads Together campaign, Prince William, Kate Middleton, and Prince Harry broke not only with the British royal family's long-standing tradition of keeping their personal and emotional struggles to themselves, but also with the general societal tradition of sweeping mental illness under the rug. Though the royals have since been praised for making those paradigm-shifting breaks, at the time, they couldn't get a single celebrity to partner with Heads Together, as William revealed during a panel at the Davos World Economic Forum on Wednesday.

"There was this sort of elephant in the room, if you like, that had never been grappled, and it crossed between all the different sectors and all the different organizations that we were working with—and mental health was the thing. We realized no one was talking about it, no one wanted to talk about it," he said of their decision to start the initiative. "What was very interesting from when we set up the campaign was that not one celebrity wanted to join us. Not one person wanted to be involved in the mental health campaign Heads Together.... We rang out to a lot of people and nobody, before we started, was interested in being part of Heads Together because it was mental health."

He continued, "That was three years ago, you know, and that was a big deal. And then, obviously, once we started getting the ball rolling, once we started showing a lot more of what we were going to do, people realized that Catherine, Harry, and I put our necks on the line here; that, 'Actually, maybe it was okay, we could join.'"

William went on to speak about his own struggles with mental health. "When I started feeling issues myself, it was from my air ambulance work. I was dealing with a lot of trauma on a day in, day out basis, stuff that your body is not programmed to deal with, there's just no way it is," he said, noting, "For some reason, we're all embarrassed by emotions—British people, particularly—we're very embarrassed by revealing our emotions."

In the nearly three years since launching Heads Together, William, Kate, and Harry (Meghan Markle became a patron of the charity after marrying Harry, but has yet to attend any Heads Together–related events since then) have worked tirelessly to destigmatize mental illness. While William has spoken several times before about the trauma he experienced during his time as an air ambulance pilot, Harry recently opened up for the first time about how his mother Princess Diana's death impacted his own mental health, and Kate has led multiple discussions about the struggles facing both parents and their young children.

Related: Prince William Speaks Out About Cyberbullying, Catfishing and Photoshopping