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The Glee star explained that she found it difficult to land roles by her late teens, having previously starred in the likes of The Fresh Prince of Bel-Air and Baywatch as a youngster.
"I am half Puerto Rican, a quarter German and a quarter black," she told MTV's This is How I Made It.
"That was always a big issue for me - being mixed-race - because casting directors tended to be very like, 'Okay, are you Hispanic for this role? Or is she going to be African American?'
She added: "So, it was really tough and work started to slow down. From the time I was 16 to really up until turning 21, the roles were really, really few and far between.
"I had people say that I just wasn't a good singer. They didn't know what to do with me; I would never fit in any markets. I almost quit acting altogether."
Rivera eventually landed the role of Santana Lopez in the Fox musical comedy Glee in 2009, and has since signed a record deal with Columbia.
Glee co-star Lea Michele recently denied that she is pregnant by boyfriend Cory Monteith.
