"Their marriage has been a disaster for more than a decade," revealed a source close to the troubled couple.
An insider added: "The relationship has faced its challenges, including Eric's wandering eye over the years."
Both Gayheart, 46, and Dane, 45, have admitted to substance abuse problems. Each has spent time in rehab.
The two wed in 2004, three years after Gayheart pleaded no contest to vehicular homicide charges after she struck and killed a nine-year-old boy.
In 2009, the couple was embroiled in a sex scandal when a leaked video showed a woozy Dane — then at the height of his fame as Dr. McSteamy on Grey's Anatomy — in a hot tub session with Gayheart and a former teen beauty queen.
Two years later, Dane sought treatment for addiction to prescription painkillers.
Last year, the actor checked himself into rehab under a gloomy cloud of "clinical depression," and took a break from shooting TV's The Last Ship.
Gayheart cited "irreconcilable differences" in divorce papers filed on Feb. 16.
The former Beverly Hills, 90210 star is seeking spousal support and joint custody of the couple's daughters, Billie, 8, and Georgia, 6.
We pay for juicy info! Do you have a story for RadarOnline.com? Email us at firstname.lastname@example.org, or call us at (866) ON-RADAR (667-2327) any time, day or night.