Queen Elizabeth II dropped her guard to reveal exactly how she felt about Princess Diana's death in a revealing letter to one of her aides, it has emerged. In a newly discovered message to Lady Henriette Abel Smith, the grieving monarch described her former daughter-in-law's passing as a "huge loss to the country" and admitted that she was still struggling to come to terms with the news.

Diana died as a result of injuries sustained in a car crash in the Pont de l'Alma road tunnel in Paris on 31 August 1997. In the wake of the tragedy, many people criticised the Queen's impassive response to the tragedy despite her rare address to the nation in which she said: "[Diana] was an exceptional and gifted human being. In good times and bad, she never lost her capacity to smile and laugh, nor to inspire others with her warmth and kindness."

ABC now reports that the Queen's handwritten letter to her lady in waiting, penned just six days after Diana's death, finally sheds some light on how she really felt about watching her grandsons, Prince William, then 15 and Prince Harry, then 12, mourning their mother and her own personal turmoil.

"It was indeed dreadfully sad, and she is a huge loss to the country. But the public reaction to her death and the service in the Abbey, seem to have united people round the world in a rather inspiring way," Queen Elizabeth II wrote." William and Harry have been so brave and I am very proud of them."

She went on to detail the weight of grief that engulfed the royal family that stopped them functioning. "I think your letter was one of the first I opened -- emotions are still so mixed up, but we have all been through a very bad experience!"

Both William and Harry have made no secret of their struggle to deal with the grief in the aftermath of her death. Harry recently admitted that he sought professional help after his life descended into "total chaos".

Harry recently recalled the heartbreaking moment he walked in the funeral procession in an interview with Newsweek's Angela Levin, and implied that his immediate family may have made the wrong decision.

"My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television," he lamented. "I don't think any child should be asked to do that, under any circumstances. I don't think it would happen today."

Queen Elizabeth II
16 April 2017: Queen Elizabeth II leaves after attending the Easter Sunday service at St George's Chapel at Windsor Castle in Windsor, England Jonathan Brady/Getty Images