“What a refreshing surprise she is!” Oprah Winfrey tweeted after interviewing pop superstar Rihanna in Barbados. And now we know why. In a sneak peek of the interview, which is part of an upcoming special that will air on OWN, the 24-year-old singer gets emotional, surprising herself even, while speaking with the media mogul about her past relationship with Chris Brown and her dealing with fame. “It was embarrassing. It was humiliating,” Rihanna says when Oprah asks about the end of her controversial relationship with Brown, who pleaded guilty to physically assaulting her in 2009. “I lost my best friend.” After the very public split, Rihanna admits, “I was resentful. I held a grudge. I was dark. “During the interview, Rihanna also opens up about her struggle to cope with the pressure that comes with fame, saying she was “super-duper afraid of the pedestal” that comes with it. Rihanna’s interview with Oprah airs Sunday, August 19, at 9 p.m., on OWN.