While both Princess Diana and Prince Charles tried to keep the details of their tumultuous marriage under wraps, as public figures, that proved to be almost impossible.

Divulging information about the family dynamic is greatly frowned upon if you're a royal, so the public has had to depend on sneaky insider sources, who are almost always eager to share.

Recently, an emotional letter that revealed how Diana felt following her separation from Charles has recently been uncovered, 23 years after it was written.

The handwritten note, which was addressed to a woman named Erika, appears to be a reply to a letter she previously sent to the late Princess of Wales, discussing the heartbreaking end of her marriage.

"Our understanding is that Erika was a member of the public who was experiencing a troubled time and she wrote to Diana for advice and support as she herself had experienced a difficult time with the break up of her marriage," said Martin Nolan of Julien's Auctions.

"Diana is very gracious and compassionate, and she offers great encouragement to Erika that there will be light at the end of the tunnel. It seems as if she is speaking from her own personal experience when she writes this."

"I am thinking of you so much, hang on in there Erika."

Here's the touching letter in full:

"Dearest Erika, I have received your letter and of course read it with concern and a wish to support you in any way I can.

There is a great deal of pain inside you, hence the block as you call it - that is a natural response for someone who has been through the traumas you've experienced.

Reading between the lines of your letter I feel a very special lady is struggling with a low self-esteem because of all the bruises you had to deal with, mentally and physically.

I do know that it takes great courage to write as you did and with that honesty and clarity the light will appear in your tunnel.

"I am thinking of you so much, hang on in there Erika.

Lots of love from Diana x."

It is believed that Erika, who remains unidentified, was inspired by Diana's Panorama interview, during which she opened up about her failed marriage. She revealed to BBC's Martin Bashir that there were "three people" in her marriage, referring to her ex-husband's affair with Camilla Parker Bowles, who he eventually married.

The 1995 interview aired just days before the princess received the moving letter, which has a current starting bid of nearly $7,000.

Diana's letter to Erika is one of several she penned over the years to those in need of encouragement while going through hard times. She also wrote to her close friends and family when things in her life became difficult.

"Charles plans to kill me"

In 2007, a controversial letter in which Diana alleged that Charles planned to kill her was shown at an inquest into her death at the Royal Courts of Justice in London.

In the handwritten note addressed to her butler Paul Burrell, Diana revealed that the Prince of Wales wanted her dead so he could marry Prince William and Prince Harry's former nanny, Tiggy Legge-Bourke.

She wrote:  

"I am sitting here at my desk today in October, longing for someone to hug me and encourage me to keep strong and hold my head high."

"This particular phase in my life is the most dangerous - my husband is planning 'an accident' in my car, brake failure and serious head injury in order to make the path clear for him to marry Tiggy. Camilla is nothing but a decoy, so we are all being used by the man in every sense of the word."

Despite the evidence, Diana's friends, including Lucia Flecha da Lima, the wife of the former Brazilian ambassador to London, said the princess never feared for her safety.

"I still don't believe in it," she said. "Paul Burrell was perfectly capable of imitating Princess Diana's handwriting. I don't believe she was fearing for her life, especially from Prince Charles, the future king of your country."

We'll never really know the whole truth, but these letters are proof that being married to Charles was not easy at all.