And now, one royal expert has said that the Princes "would trade it all to have her back".
When Diana was tragically killed in a car accident in Paris on August 31 1997, Prince William was 15 years old, while Harry was just 12.
Almost 22 years on from their mother's death, both William and Harry have spoken about their struggles with grief as part of their Heads Together campaign, which encourages young people to speak openly about mental health.
Speaking about the campaign, historian Kate Williams told Yahoo UK that the Princes had suffered "a lot of mental pain", something echoed by royal expert Victoria Arbiter, who told Yahoo that the boys "would trade it all to have her back".
“We heard the Royals talk about what they had suffered, they made it clear they can suffer a lot of mental pain," said Williams.
“When you say they want to nothing, we’re talking about materialism there, what they want is a mother and that’s something that was taken away.”
Both Harry, 34 and William, 36, have spoken about their suffering, with William revealing he still misses Diana every day.
“Time makes it easier. I still miss my mother every day — and it's 20 years after she died," Prince William said in 2016.
"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 GQ a year later.
Prince Harry, speaking about the years that followed his mother's death, revealed in a documentary that he spent a long time "[not wanting to] be Prince Harry".
"Years after I spent a long time (of) my life with my head buried in the sand, you know, thinking 'I don't want to be Prince Harry, I don't want this responsibility, I don't want this role, look what's happened to my mother, why does this have to happen to me'," he said.
"But now all I want to do is try and fill the holes that my mother has left, that's what it's about for us, is trying to make a difference, and in making a difference making her proud."
In a separate interview with ITV in 2010, Harry revealed that whatever he and William do, they wonder what Diana would be doing if she "was with us now".
"Every day, I know I do and I’m sure William does as well, whatever we do, wherever we are and whoever we’re with I particularly always wonder what she'd think, what she’d be doing if she was with us now," he said.
“If she’d be sitting here having a laugh, whether she’d be in the background sticking her tongue out or whether she’d be playing football with the children.
"That’s what keeps us going every day – that thought of what would she be like if she was around today.
"It’s refreshing because we both have our own opinion of what she’d be like and ‘mad’ would be one word to describe her.”
Earlier this week, we told you 'upset’ Prince Harry ‘complained’ William was always the centre of attention, royal biographer claims.
We also revealed pregnant Meghan Markle tried to avoid cradling bump to appear ‘confident and resilient’ amid dad drama, body language expert claims.
And we showed you a rare throwback snap of Princess Diana with her young sons.
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