Prince William has revealed that becoming a father brought back painful emotions he felt following his mother's death when he was 15 years old.
The Duke of Cambridge, who has three children with his wife Catherine, said feelings from a "traumatic" event can resurface when becoming a parent.
He told a BBC documentary on mental health that he found things "overwhelming" at times.
His mother, Diana, Princess of Wales, died in a car crash in Paris in 1997.
Prince William, father to Prince George, Princess Charlotte and Prince Louis, said that he and Catherine support one another during those difficult times.
He was speaking to former professional footballer Marvin Sordell for the programme focusing on men's mental health and football.
Ex-Bolton Wanderers striker Sordell described becoming a father as "the hardest time in my life", adding that he struggled with his emotions and found it challenging as he did not grow up with a father.
The Duke of Cambridge responds: "Having children is the biggest life-changing moment, it really is.
"And I agree with you, 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 - your emotions come back in leaps and bounds because it's a very different phase of life.
"And there's no one there to, kind of, help you, and I definitely found it very, at times, overwhelming."
Prince William said that emotions can "come out of the blue" which you do not expect or think you have already dealt with.
Speaking about parenthood, the royal added: "It's one of the most amazing moments of life but it's also one of the scariest."
Prince William and his younger brother Prince Harry have, through their Heads Together mental health campaign, spoken increasingly about the impact their mother's death had on them.
Football, Prince William And Our Mental Health will be broadcast on Thursday 28 May at 20:05 BST on BBC One.