"I would like to have had her advice. I would love her to have met Catherine and to have seen the children grow up. It makes me sad that she won't, that they will never know her," he told the magazine.
Prince William and his brother, Prince Harry, were 15 and 12 years old, respectively, when Princess Diana died at age 36 in July 1997. Prince William spoke to British GQ ahead of the 20th anniversary of his mom's death.
"I am in a better place about it than I have been for a long time, where I can talk about her more openly, talk about her more honestly, and I can remember her better, and publicly talk about her better," the royal said.
"It has taken me almost 20 years to get to that stage. I still find it difficult now because at the time it was so raw. And also it is not like most people's grief, because everyone else knows about it, everyone knows the story, everyone knows her," he added.
Prince William married Middleton in 2011, and welcomed Prince George and Princess Charlotte in 2013 and 2015, respectively. He and Prince Harry spoke to Middleton about their mom's death in a video promoting mental health in April.
"We have been brought closer as a result of the circumstances," Prince William said of himself and his brother. "You are uniquely bonded because of what we've been through. Even Harry and I though, over the years, have not talked enough about our mother."