Kate Middleton's Cancer Announcement Video 'Took a Lot' as She's 'Inherently Shy' (Exclusive)

The Princess of Wales was "sincere, dignified, poised" as she shared her health update with the world, says royal biographer Sally Bedell Smith

Kate Middleton overcame her "inherently shy" nature to deliver some difficult news.

In a video released on March 22, the Princess of Wales told the world that she was undergoing treatment for cancer discovered in post-operative tests following her January abdominal surgery.

Royal biographer Sally Bedell Smith tells PEOPLE that Princess Kate, 42, was "sincere, dignified, poised, and she was forthright" in her delivery, but the announcement likely wasn't easy.

"She is inherently shy, and for her to do that took a lot," Bedell Smith adds.

While the health update was extremely personal, Princess Kate acknowledged the many others who are also facing cancer, echoing something the late Queen Elizabeth may have done.

"When she ended with 'You are not alone,' it rang a bell with something that Queen Elizabeth would have said," Bedell Smith explains. "It was very similar to what Queen Elizabeth said during COVID when she said, 'We are all in this together.' "

Related: Why Kate Middleton Announced Her Cancer News to the Public When She Did

"She was mindful there are very few people who read about that or watched that who doesn’t know someone who has cancer. She was mindful of that," the journalist and biographer continues. "It was a way of reassuring people not only about her condition but helping them have courage."

"There was not a trace of self-pity. She projected honesty," Bedell Smith says. "It was the right balance."

In addition to writing "every word" of the announcement herself, the Princess of Wales also opted to appear on camera alone, without even her husband, Prince William, by her side.

"It is a message from the princess about her health, and she wanted to personally deliver the message on her own," a palace source tells PEOPLE, adding that the Prince of Wales, 41, has been "supporting her throughout" behind the scenes.

"William is extremely proud of his wife for the courage and strength she has shown not just this week but since her surgery in January," the source says. "Now more than ever, he's focused on ensuring his wife has the privacy she needs to fully recover and that his children are shielded from the understandable interest in the news that has been shared."

In her emotional video, Princess Kate thanked well-wishers for their "wonderful messages of support" amid her surgery recovery during she called "an incredibly tough couple of months for our entire family."

"In January, I underwent major abdominal surgery in London, and at the time, it was thought that my condition was non-cancerous," the royal mom of three explained. "The surgery was successful. However, tests after the operation found cancer had been present. My medical team therefore advised that I should undergo a course of preventative chemotherapy, and I am now in the early stages of that treatment."

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Related: Kate Middleton's Cancer Announcement Video Receives Editor's Note on Getty — Here's Why

Calling the news a "huge shock," Princess Kate said that she and Prince William "have been doing everything we can to process and manage this privately for the sake of our young family. As you can imagine, this has taken time. It has taken me time to recover from major surgery in order to start my treatment."

"But, most importantly, it has taken us time to explain everything to George, Charlotte and Louis in a way that is appropriate for them, and to reassure them that I am going to be ok," she said referring to her three children: Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5. "As I have said to them; I am well and getting stronger every day by focusing on the things that will help me heal; in my mind, body and spirits."

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