• The 31 year old said he only began speaking about the loss three years ago
• Both he and Prince William didn't want to upset their father - and Camilla.
Thirty-five years ago this week, London was en fete, and the rest of the United Kingdom and much of the world came to a halt as Lady Diana Spencer promised to love, honour - but not obey - Prince Charles.
Her vows - like her wedding dress, her hair and even her virtue - were under global scrutiny. No wonder she fluffed her lines and muddled the Prince's names, calling him 'Philip Charles' instead of 'Charles Philip'.
In hindsight, the mix-up seems another omen for a doomed marriage, along with the Prince of Wales's notorious "whatever in love means" remark on the day of their engagement.
This year, as in years past, there has been nothing to distinguish the date, July 29 - but then, there has been precious little recognition, anyway, of the contribution Diana made to the Royal Family and to the life of the nation.
The rush to commemorate her in the aftermath of her death at the age of 36 in a Paris car crash (she would be 55 now) is but a distant memory. Only the most devoted of the Princess's supporters know of the Diana memorial walk, an 11km trail through four of London's parks, marked with 90 plaques in the ground; or of the Diana memorial playground with its wooden pirate ship in Kensington Gardens.
There is also, of course, the memorial fountain - derided by some as a glorified ditch - in Hyde Park, where tourists cool their feet on hot summer days. But more of that later.
This erasing of Diana by a vengeful Establishment has been a powerful narrative, aided in part by the near silence of the royals.
But this week, Prince Harry, who together with his brother William have been the finest and most visible memorials to the late Princess, indicated that the years of silence are at an end.
He spoke poignantly of his regret of not opening up sooner about how his mother's death affected him.
He was just 12 when she died. At a barbecue held earlier this week for the UK mental health campaign Heads Together, Harry, 31, said he only began speaking about the loss three years ago.
He added: "It's okay to suffer, as long as you talk about it. It's not a weakness. Weakness is having a problem and not recognising it and not solving that problem."
His remarks, which were welcomed by charity chiefs for his willingness to break the stigma surrounding mental health issues, came just days after he had been praised by the Terrence Higgins Trust for taking an HIV test live on Facebook.
The charity, which campaigns to promote good sexual health, greeted it as a "ground-breaking moment in the fight against Aids", and it was likened to the historic occasion when Princess Diana first shook hands, without wearing gloves, with Aids victims in Middlesex Hospital nearly 30 years ago.
But while his actions were being celebrated as examples of Harry's vital qualities in championing taboo subjects - and of his transformation from Prince of Knaves to Prince of Hearts - it was his reflections on his mother that were being talked about in Buckingham Palace corridors this week.
Long-time courtiers were left wondering about the significance of the remarks and why, almost 19 years after Diana was killed in that high-speed car accident, Harry should now feel the time was right to "open up" about her.
Some, perhaps cynically, were suggesting that he may have a "guilt complex" over the manner of Diana's airbrushing from royal history.
He can hardly be blamed for that, although over the years, her two sons chose to do little in the way of public gestures towards her memory.
In 2000, he and William declined to attend the opening of the Diana playground, so close to their old home at Kensington Palace. And until a decision was taken over the so-called fountain or water feature, they took scant interest in the debate over a suitable permanent memorial.
This, of course, should not be interpreted as an unwillingness to mourn their mother - they missed her desperately - but rather a determination to keep their grief and their loss private. However, there was one other over-riding factor, a dread of upsetting their father - and Camilla.
"For a long time, they worried, and those around them worried, too, that whatever they said about their mother could be misinterpreted as somehow being critical of the Prince of Wales," recalls an aide.
"The whole Diana subject was tricky, and their sensitivity about saying anything was noticeable.
"They were often upset when their mother's name cropped up, especially when books about her came out, but there was this reluctance to say or do much."
A friend of the Princess's remembers Harry's attitude as a teenager.
"He used to throw a fit if anyone mentioned his mother's name," she says. "At school, they could manage it up to a point by restricting access to television, and, of course, at home with Charles at Highgrove there were never any newspapers around."
Some suggest that this anger was because he felt something so private and personal as the death of his mother had been commandeered by the rest of the country.
A former girlfriend of the Prince says that Harry had "a lot of emotional baggage" and that he was "very sensitive". In that way, says the girlfriend, he is "terribly like Diana".
From time to time, he has spoken about his mother, albeit warily.
As early as 2002, when he was 18, he said he wanted to do something that evoked memories of her charity causes - which, 14 years on, he has substantially achieved.
A year later, when he was establishing his Sentebale charity for children in the African kingdom of Lesotho, Harry was clear that he wanted to carry on his mother's work and to "make her proud".
Recalling his night-time visits with William and the Princess to homeless projects and to visit sick children in hospital, he said simply: "I believe I've got a lot of my mother in me."
Only once has he truly abandoned caution, and that was in 2007, when he and William marked 10 years since Diana's death with a pop concert at Wembley and a memorial service at the Guards Chapel at Wellington Barracks in Central London.
The concert was held on what would have been her 46th birthday, the service on the anniversary of her death. Harry delivered a moving tribute.
"William and I can separate life into two parts," he told the 450-strong congregation.
"There were those years when we were blessed with the physical presence beside us of both our mother and father. And then there were the 10 years since our mother's death.
"When she was alive we completely took for granted her unrivalled love of life, laughter, fun and folly. She was our guardian, friend and protector." She was, he declared, "quite simply the best mother in the world".
Yet the memorial left some wondering why an occasion to honour and celebrate Diana's life left a sour taste of score-settling.
Suspicions arose when many of the people close to the Princess were excluded from the guest list - which, until a last-minute decision not to attend, included the Duchess of Cornwall.
At the time it was said those not invited, who numbered Paul Burrell the Princess's former butler, her bodyguard Ken Wharfe and her private secretary Patrick Jephson, had been omitted because they had written books and betrayed confidences.
Even 10 years on, such an explanation seems unconvincing when those concerned disclosed no secrets at all while the Princess was alive.
The boys were said to have planned the twin events down to the last detail, and palace staff said the only other occasion on which they had been quite so engaged was when it came to choosing which Army regiment to join.
But since then, they have said little - although William's decision to give his wife Kate his mother's engagement ring and to include Diana as one of their daughter Charlotte's names are eloquent reminders of her.
So what has triggered Harry's admission? According to one figure who knows him well, the Prince, who left the Army after 10 years last June, has come to recognise his future.
"He realises that with his military career over, the only thing left for him is what his mother did - charitable causes.
"As to why he should be bringing up his mother's name, I suspect it is because he has a bit of a guilt complex about how she has been remembered.
"Like his mother, he has an emotional intelligence, is tactile, too, and, like her, connects easily with people.
"He is not afraid to show his humanity. Shining a light onto forgotten children around the world seems a very natural thing for him to do."
Finding a partner to share these globetrotting ambitions, however, is a different matter.
With two serious romances behind him, to Zimbabwean farmer's daughter Chelsy Davy and to actress Cressida Bonas, Harry is no closer to settling down.
In two months, he will be 32, the same age his father was when he married 20-year-old Diana Spencer. But Charles has been careful not to interfere in his son's relationships as he is acutely aware of the pressure he himself came under to marry Lady Diana.
Unlike William, whose future is predetermined, Harry's is not, and this is why he is taking the lead in ensuring that after all these years his mother is finally properly recognised.
By the time of the 20th anniversary of her death in August 2017, he wants a new memorial in place.
At the same time, several of the Princess's closest friends have also been looking at ways to mark next year's date. They have been examining the possibility of raising backing for a life-size statue of Diana or a simple bust of her head to be placed in London.
With the passage of time, these friends feel the £3.5 million ($64.m) fountain has failed to capture the spirit of Diana.
They will, however, support whatever Harry - and William - want to do, certain that Britain will finally have a fitting tribute to the Princess of Wales: a son's tribute.