The 34-year-old Duke of Cambridge paid a visit to Keech Hospice Care in Luton, England on Aug. 25, where he got emotional while speaking with 14-year-old Ben Hines, who lost his own mom to cancer a year ago.
"Time makes it easier," William said, comforting Hines. "I know how you feel, I still miss my mother every day and it's 20 years after she died."
"The important thing is to talk about it as a family," William added. "It's OK to feel sad. It's OK for you to miss her."
Princess Diana was killed in an auto accident in Paris nearly 19 years ago, on Aug. 31, 1997. William was 15 at the time, while his younger brother, Prince Harry, was 12.
Last month, Harry also opened up about his mother's death, and admitted that one of his biggest regrets was not being open about his loss and how he was coping with it.
"I really regret not ever talking about it,” Harry said during a benefit event for his mental health charity, Heads Together. "For the first 28 years of my life, I never talked about it." Check out the video below to hear more from the royal about dealing with trauma and tragedy.