One of Queen Elizabeth's most painful moments happened in 2002 and a new documentary revealed that Her Majesty learned to live with this loneliness since then.

Queen Elizabeth lost her dearest younger sister, Princess Margaret, on Feb. 9, 2002, following a cardiac arrest. The royal was 71 years old when she passed away while the Queen was 75. 

The two grew up to be each other's confidante and best friend, aside from being sisters. Royal historian Dr. Ed Owens said in the documentary, Elizabeth and Margaret: Love and Loyalty, that Princess Margaret's passing had a cataclysmic effect on Her Majesty because her sister had been her most important ally.  

Owens said that Queen Elizabeth's place on the throne was lonely as it is but she had Princess Margaret to support her for 50 years. The expert said that Princess Margaret was one of the sources of the Queen's strength as a monarch. 

The sisters knew each other's secrets, faults and fears. However, they could always depend on each other and were on the same page when it came to dedication to their duties.  

Royal biographer Ingrid Seward agreed that it was only Princess Margaret who knew exactly what the Queen felt about their royal roles. If there was one person who really understood the monarch inside and out, it was her Margaret. The expert said that the royal sisters had such an unbreakable bond despite being very different from each other in terms of personality. 

As the Queen's life was becoming less her own because of her duty to the Crown, so did Princess Margaret when she decided to focus on supporting her sister. After several failed relationships, the Countess of Snowdown shifted and did endless royal engagements by the time she was in her 50s and 60s, giving up her party girl image in the process. 

Princess Margaret's health was problematic since she lost her left lung for a much-needed surgery in 1985. The royal was a smoker since she was 15 years old. She eventually gave up the vice in 1991 but didn't give up drinking alcohol. 

In 1998, she suffered from a mild stroke, which affected her mobility. In 2001, the Queen Elizabeth's sister was in and out of the hospital after suffering more strokes. Her funeral was held on the 50th death anniversary of King George VI, their father. Her body was cremated with the ashes placed next to the King.