Prince William is opening up about how being an air ambulance pilot took an emotional toll on him.

The British royal spoke about the role he took on after serving as an RAF Search and Rescue pilot for a special episode of Apple Fitness+’s "Time to Walk" series, which premiered on Monday.

The 39-year-old began working with the East Anglian Air Ambulance in 2015 and left the role in 2017 to focus on royal duties. The eldest son of Prince Charles and Princess Diana is second in line to the throne.

"The moment I started the helicopter training, I realized that it was better than anything," said William. "It was one of those things that I just instantly took to and thought, 'This is really cool.' I really enjoy it.’"

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Prince William

Prince William, The Duke of Cambridge, as he begins his new job with the East Anglian Air Ambulance (EAAA) at Cambridge Airport on July 13, 2015, in Cambridge, England. The former RAF Search and Rescue helicopter pilot will work as a co-pilot transporting patients to hospital from emergencies ranging from road accidents to heart attacks. (Photo by Stefan Rousseau WPA - Pool/Getty Images)

However, William admitted he faced "difficult situations" while on the job.

"Seeing patients and families ripped apart on almost a daily basis, that routine, you just get into a habit of head down and get on with it," he shared.

But there was one call in particular that impacted the royal’s mental health.

"Immediately it became clear that this young person was in serious difficulty, sadly been hit by a car," William recalled. "And of course there are some things in life you don't really want to see."

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Prince William pilot

This file picture taken on March 31, 2011, shows Britain's Prince William at the controls of a Sea King helicopter during a training exercise at Holyhead Mountain, having flown from RAF Valley in Anglesey, north Wales.  (JOHN STILLWELL/AFP via Getty Images)

"And all we cared about at the time was fixing this boy," he continued. "And the parents are very hysterical, as you can imagine, screaming, wailing, not knowing what to do, you know, and in, in real agony themselves. And that lives with you."

William’s team was able to stabilize the young boy. But the royal admitted the experience stayed with him.

"I went home that night pretty upset but not noticeably," he shared. "I wasn't in tears, but inside, I felt something had changed. I felt a sort of, a real tension inside of me."

William attempted to continue his work without processing his emotions. However, the experience came back to haunt him.

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Prince William

Prince William detailed how his mental health was impacted while on the job. (Photo by Indigo/Getty Images)

"It really hit me weeks later," he explained. "It was like someone had put a key in a lock and opened it without me giving permission to do that. I felt like the whole world was dying. It's an extraordinary feeling. You just feel everyone's in pain, everyone's suffering. And that's not me. I've never felt that before."

"My personal life and everything was absolutely fine," he continued. "I was happy at home and happy at work, but I kept looking at myself, going, 'Why am I feeling like this? Why do I feel so sad?' And I started to realize that, actually, you're taking home people's trauma, people's sadness, and it's affecting you."

William described how crucial it was for him to openly share what he was feeling behind closed doors.

"I was lucky enough that I had someone to talk to at work in the Air Ambulance because mental health where I was working was very important," he said. "Talking about those jobs definitely helped, sharing them with the team, and ultimately, in one case, meeting the family and the patient involved who made a recovery, albeit not a full recovery, but made a recovery."

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Prince William is determined to raise awareness on mental health. (Getty Images)

Today, William said he’s determined to raise awareness on mental health. It’s a cause he continues to be very passionate about.

"We know mental health has been a taboo and a stigma for a long time all around the world," he said. "And it still is. I'd like to think, in the U.K. here and the U.S., it's much more talked about, and it's opening up. But there's still a deep-rooted fear of understanding it."

"And we all need to go through a process of understanding why rather than just give in to those feelings and say, 'Listen, it's me. I'm the problem,’" he shared. "It's not. It really isn't you. And you're not alone, and it's OK. It's about what you do next. It's about having that boldness and that openness and that strength to go, 'It's going to be a long journey. It's not going to be easy, but I'm going to get there.'"

Earlier this year, William teamed up with Apple Fitness+ to record one of their audio walking experiences. The series features personal stories from well-known personalities that are aimed to encourage people to walk more.

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Prince William Kate Middleton 10 year anniversary photos

Prince William shares three children with Kate Middleton. (Chris Floyd/Camera Press/PA via AP)

William took listeners on a walk through Queen Elizabeth II’s Sandringham Estate in Norfolk, past St. Mary Magdalene Church and eventually in Anmer, where he and his wife Kate Middleton share a country home with their children.

William, who has used his platform to raise awareness on mental health, chose three charities to receive a five-figure donation from Apple: Shout in the UK, Crisis Text Line in the USA and Lifeline in Australia, People magazine reported. According to the outlet, Crisis Text Line and Shout provide free, 24/7 confidential support for people in crisis via text, and Lifeline provides free 24-hour confidential crisis support and suicide prevention services.