Charles Spencer on Whether He Told Sister Princess Diana About His Boarding School Abuse (Exclusive)

The 9th Earl Spencer confronts his childhood trauma at Maidwell Hall in his new memoir, 'A Very Private School'

Charles Spencer never told his sister Princess Diana about the abuse he endured at boarding school.

The historian and younger brother of the late Princess of Wales reflects on his harrowing experience at Maidwell Hall in his new memoir, A Very Private School, exclusively excerpted by PEOPLE in this week’s issue. Though Spencer was exceedingly close with Diana in childhood, he tells PEOPLE that neither she nor his other sisters knew what he suffered starting at age 8.

"I don’t remember us ever really discussing what we were going through. She went to a very gentle place, I believe," the 9th Earl Spencer says of Diana’s own boarding school years. "We would reconnect for the school holidays, and I don’t remember ever talking about it with her."

When Spencer, 59, finally told his older sisters, Sarah and Jane, 18 months ago, "they were stunned and appalled."

Related: Charles Spencer Reveals Childhood Trauma in Heartbreaking Account of His Boarding School Experience (Exclusive)

In the heartbreaking new memoir, which was published on Tuesday, Spencer confronts the childhood trauma of his harrowing five years at an elite institution in the English countryside that left its young students vulnerable to predatory adults.

It was a difficult book to write, plunging Spencer — who reveals for the first time that he was the victim of physical, verbal and sexual abuse at the school — into a painful past.

"I had always felt that I had dealt with this chapter of my life and had put it to bed," says the father of seven, who has been married to wife his Karen since 2011. "But talking to classmates and hearing their experiences was very triggering for me. I'd be very struck by the horror of what they had shared. And that was the hardest bit."

Related: Charles Spencer Shares Rare Childhood Photo with Sister Princess Diana and Their Mom

At his lowest points in childhood, Spencer reveals in the memoir, he contemplated self-harm.

"I genuinely did consider self-inflicted wounds with my father’s shotgun very regularly at the end of term," he says. "No child should have to think of that, at the end of holidays, end of vacation. I would really strongly contemplate that just to stop myself having to go back to that place."

Today, he worries that in sharing his story, some will judge his candor as a plea for pity from those born into tremendous wealth and privilege.

"My biggest fear is that people might think, 'Poor little rich boys. What have they got to complain about?' It’s not about the wealth. It’s about vulnerability as children in any social class. My biggest hope is that it genuinely will help people," Spencer tells PEOPLE.

If you suspect child abuse, call the Childhelp National Child Abuse Hotline at 1-800-4-A-Child or 1-800-422-4453, or go to www.childhelp.org. All calls are toll-free and confidential. The hotline is available 24/7 in more than 170 languages.

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