In the October issue of Shape, LeAnn Rimes explains why she cheated on husband of six years, Dean Sheremet.
"My relationship with Dean was great, but ultimately it wasn't a fulfilling marriage for either of us," says the singer, 28. "We got married so young: I was 19 and he was 21, so as we got older, we grew apart."
Us Weekly first broke the news that she had an on-set fling with actor Eddie Cibrian while shooting a Lifetime movie in late 2008.
Rimes -- who now lives with Cibrian, 37, and his two sons, Mason, 7, and Jake, 3 -- isn't proud of her behavior.
"I understand why people are disappointed in me, especially since I grew up as America's sweetheart," she says.
She later adds, "I think any relationship is hard to get out of, and I don't think the way I did it was right."
The controversial star says after her fans learned of her affair she got "so depressed."
Recalls Rimes, "I didn't want to get out of bed."
Looking back, she says, "I truly believe there are lessons in it for me to learn. Cultivating strength from rough situations is the most important thing. After going through this, I know I can face anything."
She also credits exercise with helping her snap out of it.
"Just getting to the gym -- even for a few minutes -- was a big accomplishment," says Rimes, who appears in a bikini on Shape's cover.
Working out "really did make me feel better and kind of saved me," she adds. "It gave me a bit of sanity. Even now, on days when I'm really frustrated, I'll walk into the gym and say to my trainer, 'We're boxing today. For an hour. I need to hit things.'"
She and Cibrian now go to the same gym.
"Sometimes we take group-cycling classes together, but when it comes to weight training, we pretty much do our own thing," she says. "He's going to kill me for saying this, but when I ask him to work out with me, he always says no. And when I ask why, he says, 'Because your workouts are too hard.' And he's right: I'm serious about my exercise. I don't just doodle around. I see people at the gym talking on their cell phones. If they think their bodies are really going to change ... I mean, you've got to put in the effort!"
She says she feels like her body is "changing" and that she is "growing up into a woman. I probably weigh about 6 pounds more than when I shot that cover, but my body fat is the lowest it's ever been. I've gained a lot of muscle, and I feel like I have more curves too."
Her favorite body art? "My butt--well, I've always had a butt--but now I definitely have a butt, which I love."