The Queen was once told to be "brave" by her mother in a bid to boost her confidence.
Lady Penn, a former lady in waiting to the Queen Mother and a friend of the Queen, revealed that a young Princess Elizabeth - now the nation's longest reigning sovereign and a composed head of state - used to find walking into a crowded room daunting.
But her mother urged her to step through the door as if she was in charge.
Lady Penn appears in a new Channel 5 documentary series, Elizabeth: Our Queen, which begins on Tuesday to coincide with the 66th anniversary of the Queen's accession to the throne.
Princess Elizabeth became Queen on February 6, 1952 following the death of King George VI.
The 25-year-old was thousands of miles away in Kenya on a Commonwealth tour with the Duke of Edinburgh at the time.
Gun salutes will be fired to mark Accession Day including a 41-gun salute by the King's Troop Royal Horse Artillery in London's Green Park and a 62-gun salute by the Honourable Artillery Company at the Tower of London.
As she does most years, the Queen will reflect on the anniversary of her father's death in private at Sandringham, where she has been staying for her annual winter break.
Lady Penn said in the documentary: "The Queen Mother told the Queen when she was very young to be brave.
"I think the Queen probably when she was young felt walking into a room full of people was rather daunting.
"So she said to her 'What you want to do, when you walk into a room, walk through the middle of the door'.
"And I think by that she meant, don't sort of go in apologetically. You walk through as if 'I'm in charge'.
"I think that was very good advice."
The first episode also tells how Princess Elizabeth learned of her uncle Edward VIII's abdication in 1936 from a footman after she returned from a swimming lesson.
She heard noises outside the palace and asked what they were, only to be told her uncle was no longer monarch and her father was now king.
The 10-year-old princess had become the heiress presumptive.
- Episode one of Elizabeth: Our Queen will be screened on Channel 5 on February 6 at 9pm.