Patrick Jephson said that the two Princes bear a “heavy burden of expectation and responsibility” as they grow older than their mother was when she died. “For them, every new morning is one more than she was allowed,” he wrote in the Daily Mail.
Prince William, 39, and Prince Harry, 36, reunited on July 1 at the Diana statue unveiling ceremony in the Sunken Garden in Kensington Palace.
It was the first time the pair had met in public since Prince Philip’s funeral in April - and seen by many as an ideal opportunity to heal their reported rift.
The two Princes put their differences aside during the ceremony to honour their mother’s memory.
They shared jokes and laughed together and were seen smiling as they officially unveiled the statue.
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In the beginning of 2020, Duke and Duchess of Sussex announced that they were stepping back from royal duties.
"Today, Diana’s sons are shaping their own lives and following their own interpretations of Royal duty,” Mr Jephson wrote.
"Time flies: they are both older than she was when she died.
"That’s a heavy burden of expectation and responsibility.
“But it’s an opportunity, too… as they surely don’t need reminding."
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Diana's former private secretary also commented on the statue honouring Princess Diana.
Prince William and Prince Harry commissioned the statue in 2017 to honour what would have been their mother's 60th birthday.
The statue, unveiled on July 1, divided critics.
"Diana was not an easy person to define and nor is her legacy,” Mr Jephson said.
"If our memories and appreciation of Diana begin and end with her love of children – as the committee appears to be encouraging us to believe – then we do both her and ourselves a disservice."