Princess Diana’s younger brother, Charles Spencer, has revealed that he sought help at a “residential treatment” center for trauma after revisiting childhood abuse and suffering a “breakdown” while writing his new memoir A Very Private School, RadarOnline.com has learned. “It took me into very dark places inside of me. I had endless nightmares. I didn’t know the appalling things, the really serious things that had happened to some of my friends and contemporaries,” the 9th Earl Spencer told the BBC in an interview that aired on Sunday. “And I’d come back from having met them and interviewed th…