In a new single, "Borrowed," the star opens up about the couple's forbidden love, which developed on the set of Northern Lights while both were still involved with other people. (Rimes wed Dean Sheremet in 2002; Cibrian had been married to Brandi Glanville since 2001.)
"I wanna hold out, but I hold on, even though I know it's wrong," she sings. "Cause I don't want to give you back, you're the best I ever had -- but you're borrowed."
"I carried that song and that title around with me for about eight months," Rimes told Extra after her X Factor performance with Carly Rose Sonenclar on Dec. 20. "I was afraid to write it because I knew I had to be incredibly personal...When I did, it was through a lot of tears."
Another reason she waited so long? Cibrian. Though she told Extra her husband of a year-and-a-half is "always so supportive," she worried a little about rehashing their past.
"I actually was scared to put him in the song, because it does talk about how we both hurt each other," she explained. "There was a moment where we did have some hurt between the two of us that we had to accept and get over and move forward."
Because of that, however, their marriage is much stronger. As she told Extra, it's "made for an amazing relationship."