Winslet stuck with the relationship for a while for the children, but Mendes's tendency to be a workaholic is one of the reasons the couple split, says the source. They separated in January, but decided to keep it quiet until after the Oscars.
Winslet, 34, and Mendes, 44, who have a 6-year-old son, Joe, are no longer living together, but the source says that Mendes is still living in the same Manhattan apartment building. (Winslet also has a daughter, Mia, 9, from her first marriage to British director Jim Threapleton).
Currently, Winslet is enjoying some down time in Mexico with her brother on a "long-planned vacation," adds the source, who says rumors that the Oscar winner is romantically involved with Leonardo DiCaprio are totally false.
A movie-industry source tells PEOPLE the A-list couple's split is a surprise, calling them very "normal" and "non-Hollywood." "They seemed like they had a solid marriage and relationship," says the source. "They put as much effort into having a real life as they did on their careers."
In April, Winslet will begin filming an HBO series in New York. Winslet's role in the miniseries remake of the novel Mildred Pierce: A woman who separates from her husband and a takes a job as a waitress.
With reporting by K.A. BAKER
For more on the story behind Winslet and Mendes's split, pick up PEOPLE, on newsstands Friday