LONDON (AP) — It is an image those who saw it will never forget: Prince William and Prince Harry — just boys, really — walking silently behind their mother's cortege as the world mourned Princess Diana's death in 1997.

Now Harry has revealed for the first time that losing his mother when he was only 12 left him in emotional turmoil for 20 years, filling him with grief and rage he could only manage after he sought counseling.

Breaking sharply with the royal tradition of maintaining a stoic silence about mental health, the 32-year-old prince told The Daily Telegraph in an interview published Mon...