"I did one of the most selfish things that I possibly could do, in hurting someone else," the country superstar says in the new issue of PEOPLE, speaking for the first time about the end of her 7-year marriage last year, following an affair with married actor Eddie Cibrian.
"I take responsibility for everything I've done. I hate that people got hurt," says Rimes. "But I don't regret the outcome."
Rimes, 27, who is now in a committed relationship with Cibrian, 36 (their respective divorces will be final soon), says they became friends while shooting the TV movie Northern Lights in November 2008, and it wasn't until several months later that their relationship "tipped over."
"It was very emotionally driven," says the singer, whose new single "Swingin'" off her upcoming covers album Lady and Gentlemen is out June 14. "I never once thought what I was doing was okay."
She says she and Dean Sheremet, 29 – a former dancer who is now a chef at Nobu in New York City and has been dating photographer Sarah Silver since last year – worked on their relationship after her infidelity became public, but ultimately decided that "it wasn't a marriage that fulfilled either one of us," says Rimes.
For more on Rimes's split, her emotional year and her new life with Cibrian, pick up the new issue of PEOPLE, on newsstands Friday