The 'We Found Love' singer was speaking to Oprah Winfrey for an Oprah's Next Chapter interview, which will be screened on August 19 at 9pm ET on OWN.
Discussing her high-profile split from Brown in 2009 after the 'Don't Wake Me Up' star assaulted her, Rihanna admitted: "It was embarrassing. It was humiliating. I lost my best friend."
Talking about the aftermath, she went on: ''I was resentful. I held a grudge. I was dark.''
Rihanna also opened up about her life generally, confessing that she is "super duper afraid of the pedestal that comes with fame", and commenting: "It's amazing how lonely you can feel."
It was alleged last month that Rihanna and Brown shared an evening together on a yacht, but Brown later denied the claim.
Watch a promo for the Oprah's Next Chapter Rihanna interview below: