Prince William has never gotten over the "shock" of his mother Princess Diana's death.
The 34-year-old royal has opened up about the grief he still deals with after the tragic loss of his and his brother Prince Harry's mother, who was killed in a car crash in Paris, France, in July 1997 at the age of 36, when he was just 14 years old.
Speaking in a new BBC documentary 'Mind over Marathon', he admitted: "The shock is the biggest thing."
The Duke of Cambridge - who has three-year-old son Prince George and 23-month-old daughter Princess Charlotte with his wife Duchess Catherine - was asked by a widow of one of the marathon runners, whose son was barely a year old when his father committed suicide, how her children will cope with the loss as he was much younger than William and Harry, 32, were when Diana died.
She asked: "When your mum passed away you were a bit older than my children. I worry about them growing up. They'll be OK won't they?"
William assured her: "They'll be absolutely fine. With a mum like you they'll be absolutely fine."
He went on to explain how his work in mental health - running the charity Heads Together with his wife and brother - was something he felt he needed to do because of what he and Harry went through.
He added: "Like you said, the shock is the biggest thing. I have my reasons for getting involved with mental health with what happened with my mum when I was younger I still feel, 20 years later about my mother, I still have shock within me."
On how he's never fully got over his parent's death, he continued: "People say shock can't last that long, but it does. You never get over it. It's such an unbelievably big moment in your life that it never leaves you, you just learn to deal with it.
"You'll provide the blanket of security and understanding that they'll need. You doing this is a hugely positive step."
William's admission comes after Harry said he almost suffered a "complete breakdown" after Diana's death.
He said: "I can safely say that losing my mum at the age of 12, and therefore shutting down all of my emotions for the last 20 years, has had a quite serious effect on not only my personal life but my work as well.
"I have probably been very close to a complete breakdown on numerous occasions when all sorts of grief and sort of lies and misconceptions and everything are coming to you from every angle."