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November 18, 2015 09:00 AM

When Gwen Stefani discovered earlier this year that her now ex-husband Gavin Rossdale was having an affair with their children’s nanny, she was “completely blindsided,” a close Stefani source tells PEOPLE.

But while she was shocked by the betrayal (Rossdale’s rep has not commented), the couple’s 13-year marriage had already been on the rocks for quite some time.

“It was the final straw in a long list of awful behavior,” says the source, who added that the affair hit even harder because it was “taking place amidst the family in their home.”

News of the affair broke last week, but the Voice coach – who still tried to keep her family together after she found out what was happening, adds the source – has been staying upbeat and drawing support from new boyfriend Blake Shelton.

“Gwen is staying strong and focused on her children whom she is extremely close with,” says the source. “The kids are Gwen’s top priority. She has always run the household and activities and continues to do so.”

For more details on the breakdown of Gwen Stefani and Gavin Rossdale’s marriage and how she’s doing now, pick up the latest issue of PEOPLE, on newsstands Friday.

Meanwhile, Rossdale – who shares sons Kingston, 9, Zuma, 7, and Apollo, 20 months – with Stefani, has also been protecting the couple’s kids.

“He’s a great father,” says a Rossdale source. “He’s with his kids now, and he’s doing everything he can to protect them and keep things as normal as possible. He’s dealing with it as best he can.”

With reporting by JEFF NELSON

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