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Stepping out for a morning jog on Aug. 24, LeAnn Rimes was sporting more than just a sunny running ensemble. The singer—who recently split from husband Dean Sheremet—wore an ear-to-ear grin that hadn't been on display much in recent weeks. "LeAnn looked so happy," says an observer. "She was beaming."

The smiles might have something to do with her string of recent dates, including a concert two nights before, with her long-rumored beau, CSI: Miami actor Eddie Cibrian. After months of speculation about their relationship, the pair went public with a round of golf on Aug. 20 and caught an Aug. 22 Kings of Leon concert.

"She's a 27-year-old woman who's come to terms with the fact that her life is entering a new chapter," says a friend, adding that making peace with the end of her marriage to songwriter and former dancer Dean Sheremet, 28, has brought Rimes some much-needed clarity. "She's finally [realizing] Dean is going to be in her life as a friend. That's a huge weight off of her shoulders."

But the personal growth has come at a price, as both Rimes and Cibrian have seen their marriages end since their reported affair surfaced in March. Cibrian, 36, who recently separated from wife Brandi Glanville, was spotted "looking relaxed and laughing" with sons Mason, 6, and Jake, 2, at an L.A. Dodgers game Aug. 22, but his wife of eight years is reeling.

"He's just not who he said he was," Glanville, 36, tells PEOPLE. The two couples actually double-dated while Rimes and Cibrian were filming their Lifetime movie, but Glanville grew suspicious. "Girls get that way with my husband all the time. He's a good-looking guy," says Glanville. "But it was different this time. It was a little more reciprocated." Glanville says when Rimes called her recently, she told the singer, "He's all yours," and plans to file for divorce. Sheremet is also struggling, says a friend. "He is pretty heartbroken and devastated. Eight years is a long time to be with one person, so it's a big adjustment."

FOR THE LATEST ON LEANN, GO TO PEOPLE.COM/LEANN_RIMES

  • Contributors:
  • Reported by Pernilla Cedenheim/Los Angeles,
  • Jennifer Garcia/Los Angeles,
  • Jessica Herndon/Los Angeles,
  • Joey Bartolomeo/New York City,
  • Eileen Finan/Nashville.