The couple's romance came to a violent end when Brown attacked his pop star girlfriend in his car on the eve of the Grammy Awards. He was convicted of assault and placed on five years' probation.
The Umbrella hitmaker confirmed last month she is giving Brown another chance, and now he has opened up about their controversial reconciliation.
Speaking at Sir Elton John's Oscar viewing party on Sunday, he tells the Daily Mirror newspaper, "Sometimes... you fight with the one you love and things get said, stuff spirals.
"That night was the deepest regret of my life, the biggest mistake. But she loves me - what can I say? I'm forgiven... but, yes, I worked hard for it."
The singer wasn't allowed to forget the incident at the Academy Awards as Oscars host Seth MacFarlane drew gasps from the star-studded audience when he compared violent slave movie Django Unchained to a "date night for Rihanna and Chris Brown".
However, Brown insists he is used to the jibes, adding: "People attack me and criticise me, it happens all the time. But they don't know me, and they don't know us. It doesn't bother me any more. Other people can judge us but they don't know anything.
"I've learned a lot since that night and I've grown up. I was very young and I took a lot of stuff, fame, for granted. I didn't understand it. The support from those who do know me and the fans, who are incredible, got me through."