The two had a falling out while they were filming Revolutionary Road together in 2008, but reconciled shortly after.
"Working and living together was incredibly intense and basically overwhelmed their relationshipm" claims a source. "The pair have a very different work ethos - Sam wanted to switch off after a long day and Kate liked to analyze every nuance of her performance. It was just too much."
Mendes went to Florida to film Away We Go and even though they were having trouble, the couple found it difficult to be so far away from each other and pledged to spend more time together.
"After being in each other's pockets for so long, this, ironically, also proved really difficult," continued the source. "So last year they both made a commitment to spend the year together and do less work. But it actually ended up with them going a bit stir crazy."
"They are both really driven and were arguing more than ever before. Kate even did a really small amount of filming to try and ease the pressure and tension that had built up but they had a series of rows over projects this year and that was the straw that broke the camel's back," added the insider.
Friends have come forward to reveal that Kate and Sam have been living together for a long time.
"They have been living separately for a while," said a friend. "There had been some huge rows behind closed doors before they decided to formally separate."
The pair are hoping to split amicably in order to avoid a court battle for custody or splitting assets.
The source added, "Kate is devastated. She thought their marriage was forever."
[Image via WENN.]