"It happened over the last few weeks," explains the source. "She basically was just over it."
A rep for Rihanna (she's currently vacationing with family in her native Barbados) could not be reached for comment.
Los Angeles Dodgers outfielder Kemp, 26, helped Rihanna, 22, move on from Chris Brown, who infamously assaulted her in February 2009. The duo were first spotted frolicking together in Cabo San Lucas last January, and slowly opened up about their romance in the spring.
But the "Only Girl" singer has been "so busy," the insider says. "He just can't keep up with her crazy travel schedule."
"Matt's sick of always following after her like a puppy dog all over the world. He wants something more normal," the source adds.
Plus, the insider says, "It was never as serious as it looked. It was always just [about] having fun."
Still, back in May, Kemp gushed about his love: "She's just a fun person. I love to be around her."
In June, Rihanna similarly said of Kemp: "I'm so happy. I feel really comfortable and it's so easy. [The relationship] is just my peace. It keeps me sane."
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