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Eager to leave her 20s behind, LeAnn Rimes celebrated her 30th birthday 10 days early on Aug. 18 with a beachside party in Laguna Beach, Calif. Surrounded by friends and family, the country star cheerfully blew out candles on a cake decorated with a figure of herself and the words "Die, 20s, Die!" Nine days later, Rimes told PEOPLE, "My 30s are going to mean freedom. The next step in my life-a new chapter."

Before she could turn the page, there was one more thing to do: On Aug. 29, the day after her actual birthday, Rimes voluntarily entered a 30-day in-patient treatment facility. Her goal, according to a statement from her rep, was to deal with "anxiety and stress" and "to learn and develop coping mechanisms." Rimes explained to PEOPLE in an exclusive interview before checking in, "I'm happy and healthy. I just want to be happier and healthier more consistently." She added, "I don't want to wake up and not truly be enjoying my life and these amazing things around me."

One clear source of stress: Over the past few years, Rimes has taken hits, often on social media, for everything from posting pictures of her slender, bikini-clad frame (her rep confirms she is not in treatment for an eating disorder or addiction issues) to the fact that she and her husband of 16 months, Eddie Cibrian, 39, started their relationship while both were married to other people. Rimes addressed the affair in 2010 ("I take responsibility for everything I've done," she told PEOPLE. "I hope there can be some forgiveness and acceptance"), but the criticism continued.

In fact, a day after the singer entered treatment, her attorney filed a lawsuit against two women who are online supporters of Cibrian's ex, Real Housewives of Beverly Hills star Brandi Glanville, 39. Rimes alleges they invaded her privacy last spring by illegally taping a conversation with her and leaking "out-of-context" excerpts online. "These things have really hurt her, and she didn't always know how to deal with it," says a Rimes friend. "It's a lot for someone to take."

But her decision to seek treatment wasn't just about cyber-bullying. "Sometimes when I look in the mirror, I see a child, then I look and I see a woman who should be turning 60," she says. "I've always taken care of everyone else, and I think I've buried some issues for a long time." Rimes-who appeared on Star Search at 8, became a household name at 13 and four years later famously sued her father for stealing from her-says she's been in therapy for years. "I grew up having to care what people think; it was part of my job," she says. "I've been built up and torn down, built up and torn down. It's been difficult to tune people out, especially in the last few years. Now I'm starting to care more about me and not what everybody else thinks."

Cibrian has pledged his full support and, along with Rimes's close friends, will be visiting her in treatment to help her work on coping techniques. "My wife is the most remarkable and courageous woman I know," he tells PEOPLE. "I am so proud of her for having the strength to finally take some time for herself. I will be with her every step of the way." In her desire to seek help, Rimes had their future in mind. "I want to make my music and be a happy woman, a good wife, a good mom and one day hopefully have a child of my own," says the singer, stepmother to Mason, 9, and Jake, 5. "I don't ever want to bring a child into this world without being as healthy as I can be; it's as simple as that."

After just a few days in treatment, "she's already feeling better, stronger," Rimes's friend says. (The singer, who will release a new album, Spitfire, early next year, will leave the facility on weekends to fulfill tour obligations.) Rimes is hopeful that in 30 days she'll feel more in control. "All those things in my life will still be there," she says. "But I'm hoping they're not going to affect me as much."