Credit: Steve Granitz/

Kate Winslet's split from husband of seven years Sam Mendes has been a long time coming, a source tells

Apparently, the relationship first showed signs of trouble while the two -- who announced they were splitting on March 15 -- were filming Revolutionary Road in 2007.

"Sam told friends Kate would talk about the film the moment they woke up, and it was overbearing," the source says.

Things worsened when Winslet, 34, and Mendes, 44, took time off in 2009 to be together.

"They went stir-crazy and argued more than ever," the source tells Us.

The stars finally decided "it was over" when Winslet agreed to shoot a miniseries, Mildred Pierce, in New York City even though Mendes wanted to remain in London.

The source says, "Kate told Sam, 'Your career is not more important than mine."

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