"She tried to support him as much as she could, but at the end of the day she realized it wasn't best for her and her children (to stay)," someone close to Spears tells PEOPLE in its upcoming cover story. "She's standing up for herself and making positive changes in her life." (The couple have two sons: Sean Preston, 1, and Jayden James who was born on Sept. 12.)
Rumors of a Brit split were rampant: The couple hadn't been photographed together in weeks, and Spears skipped several recent events to promote Federline's album Playing with Fire. Still, the timing took many by surprise.
"As far as I knew they were perfectly fine," says Michael Prescott, a family friend from Kentwood, La., Britney's hometown. "I talked to Miss Lynne (Britney's mother) just the other day and she said Britney and Kevin were going to take a vacation trip together to Hawaii, just get away from everything for a few days."
The announcement "came as a total surprise," says a Federline pal. "We're all in shock. (We're hoping) it might just be that she's feeling a lot of postpartum emotions and this is something that will blow over."
Complete Britney Coverage
By the time a solo Federline arrived in Toronto Monday "he looked pretty depressed," says a photo agency rep who was with him.
Spears, on the other hand, had never looked happier. On Monday she dropped in on David Letterman at the Late Show looking svelte and wearing a short black dress and blonde bob.
After the appearance Spears went straight from the Ed Sullivan Theater in midtown Manhattan to the nearby Sony Studios, where she faxed her signature to the Los Angeles lawyers handling her divorce. The next day, Nov. 7, the lawyers filed divorce papers at the L.A. county superior court.
Though the couple's two-year marriage always seemed a little shaky (one Web site started a divorce countdown the day after their September 2004 wedding), a source close to Federline says the marriage took a turn for the worse in late September when Federline headed to Las Vegas with his friends to party at Tao for the nightclub's one-year anniversary, leaving Britney, who had just given birth three weeks earlier, home alone.
"This was her last straw," says the source. "He said, 'I have to go to Vegas.' He didn't have to go to Vegas. He uses any excuse to party. She said, 'I've had enough.' "
Not long after that, according to the source, Federline moved out, and has been staying with friends who have a place in the San Fernando Valley.
For more on Kevin and Britney's split, pick up PEOPLE, on newsstands Friday. Subscribers can also read the complete cover story online here.