Prince William's childhood promise to Princess Diana is heartbreaking!

It's been over 21 years since Princess Diana's tragically passed away.

While Prince William - and his brother Prince Harry - continue to melt hearts around the world with their touching tributes to the late Princess, the 36-year-old admits he's still "very angry" about her death and finds "it very difficult to talk about."

But the Duke of Cambridge's most heart-wrenching revelation following her death, is a promise he made to his mother when we was just 14-years-old.

When Princess Diana and Prince Charles divorced, there was a massive debate around whether the people's Princess should retain her title.

While the Queen was happy for her to keep it, Charles was not.

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Which meant Diana ultimately had her 'Her Royal Highness' title stripped away.

In losing her title, Diana was required to curtsey to her sons - which upset a young Prince William greatly.

So he vowed to help his mum reclaim her title.

The then 14-year-old reportedly said: "Don't worry Mummy, I will give it back to you one day when I am king."

Unfortunately Prince William never had the chance to fulfil his promise as three years later, his mother died in Paris.

We're not crying! You're not crying!