Queen Elizabeth II
Queen Elizabeth II's successful reign is due to a motherly advice from her mom. Pictured: Queen Elizabeth II is seen at the Chichester Theatre while visiting West Sussex on Nov. 30, 2017 in Chichester, United Kingdom. Getty Images/Stuart C. Wilson

Queen Elizabeth II's successful reign could be attributed to a simple piece of advice she received from the Queen Mother.

According to Lady Penn, former lady-in-waiting to the Queen Mother and the monarch's friend, Her Majesty's mom helped her grow from a shy girl into a calm, confident and poised royal. Queen Elizabeth II's mother made a great contribution to her successful leadership.

"The Queen Mother told the Queen when she was very young to be brave," Penn said (via Sunday Express). "I think the Queen probably 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,'" Penn explained.

The new revelations about the queen will be shown in a new documentary that will be broadcast on Channel 5.

Historian Kate Williams also shared that the queen was at a swimming lesson on the day of Edward VIII's abdication in 1936. When King George VI took over the throne, Queen Elizabeth II's fate to be the next monarch was sealed.

When the reigning queen and her sister realized it, Margaret told Queen Elizabeth II: "Poor you." The monarch took over the throne from her father when she was just 25 years old. On Tuesday, the queen will be celebrating her 66th anniversary on the throne. But it would not be a day of celebration for Queen Elizabeth II as it was also her beloved father's death.

In related news, the queen was pleased with BBC's documentary about her coronation and made sure to call the network to express her gratitude. In the documentary, she also shared how her father asked her to make a full review of his coronation and how heavy her crowns were.

"You can't lean down to read your speech. You have to bring [the speeches] up. Because if you did your neck would break and it would fall off," Queen Elizabeth II said. "Nothing like that is comfortable."