Dawson, who confirmed she was dating French DJ Mathieu Schreyer back in 2008, is believed to have ended the relationship due in part to wanting to focus on her career.
"They were always in separate places because of their careers," a source told Us Magazine.
It is believed that the break-up may have also been to do with Dawson's friends dislike of Schreyer.
"I know her close friends did not love his personality," the source explained. "He wasn't that smiley type of guy.
"She broke up with him," they added. "She was the one who ended it."
The actress is best known for her role as Gail in the 2005 comic book adaptation Sin City, and recently claimed that if a sequel ever gets made it will pay tribute to the late Brittany Murphy
Dawson is set to reunite with her Unstoppable co-star Chris Pine later this year in the Jamie Linden ensemble drama Ten Year.