Chris Brown and Rihanna's fairytale romance fell apart after he lied to her about a former flame.

In his new documentary Chris Brown: Welcome to My Life, the R&B star talks about his passionate relationship with the Umbrella singer, and in a clip posted on Facebook, he admits he and Rihanna fell in love quickly when they were teenagers.

But the love affair took a hit after he confessed to sleeping with a woman prior to their relationship - after assuring his girlfriend he hadn't.

"My trust totally was lost with her, she hated me after that," Chris says. "I tried everything, she didn’t care. She just didn’t trust me after that.

"From there, it just went downhill because there were too many verbal fights, physical fights as well. Mutual sides. It is the first time I get to say anything. We would fight each other. She would hit me, I would hit her and it never was OK."

Brown, now 28, admits the couple attempted to stop fighting, adding, "There was always a point where we’d talk about it like, 'What the f**k are we doing?'" he adds. "Like, 'I don’t like you slapping me. If I go on stage I got a scratch on my face and I gotta explain it like - Oh, no I fell! If you got a scar or a bruise you gotta put makeup on'."

The singer goes on to admit he felt like a "monster" every time the couple fought.

Brown and Rihanna's romance ended when he infamously assaulted her during a fight after Clive Davis' Grammy party in February, 2009.

The singer admits the row was triggered by the unidentified woman he previously had sex with approaching the couple at the event: "The ceremony's about to start, she’s just crying," Chris recalls. "She got over it. She started drinking a little bit, we both was drinking a little bit. We was tipsy, laughing and then we left."

Rihanna then saw a text message from the woman: "She starts going off, she throws the phone... She hits me a couple more times."

"I remember she tried to kick me, but then I really hit her, with a closed fist, I punched her. I busted her lip," he adds. "When I saw it, I was in shock. I was like, 'F**k, why the hell did I hit her?' From there she just spit in my face, spit blood in my face and it enraged me even more."

Brown pulled over the car in an effort to try and resolve the fight, but Rihanna took the keys from him and pretended to throw them out of the window.

"I get out the car and I'm looking for the keys and somebody yelled and she yells out her door, 'Help, he’s trying to kill me'."

Brown was charged with assault and sentenced to hard labour for his actions that night.

He and Rihanna briefly reunited at the end of 2015.