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Most of us had no doubt that Gwen Stefani and Gavin Rossdale were in it for the long haul. But on Aug. 3, we were all proven wrong; things weren't so "hella good."

Gwen Stefani pulled the plug on her 13-year marriage, putting an end to one of the most rock-and-roll couples of modern times. The couple always seemed smitten with each other, but underneath it all there was tension.

The split "was a long time coming," a source tells Us Weekly.

"It only looks sudden, but everything was worked out months ago so paperwork could be filed and the marriage could be ended quickly and easily," the source said.

The couple has three children: Kingston, 9, Zuma, 6, and Apollo, 17 months. Apparently things had been going South around the time of the birth of their most recent child.

"Things were rocky before then but they tried to make the marriage work," the source said. "But it just couldn't. A lot of the feeling was gone."

Could there have been another factor in play, too? TMZ reports that Gwen had her suspicions about her husband's faithfulness and believed that he may have been cheating on her.

The website, who initially broke the news, said distance also put severe strain on the marriage. Gwen, the outlet said, often complained about Gavin being away from home so much, touring with his band, Bush. During a recent tour stretch, he was gone from December 2014 through March 2015.

"Gwen put pressure on him to stop touring and stay home," TMZ says. "For his part, he felt it she was hypocritical because she was on the road, too."

The couple, it's been reported, had no prenuptial agreement, so their earnings are likely to be divided. They are both asking for joint custody of their three children.

"While the two of us have come to the mutual decision that we will no longer be partners in marriage," they announced in a statement. "We remain partners in parenthood and are committed to jointly raising our three sons in a happy and healthy environment."

The parents priority, everyone agrees, is their children.

"Their priority is making sure they maintain a safe and secure home for their children regardless of what's happening," a source close to Gwen told People Magazine. "When you're separated and have kids, you're much more emotionally invested."

"Knowing how she is as a mom, her top priority is making sure her kids feel secure and comforted during the process," the source added, while saying the split is amicable.

Another source echoed the family first sentiment. "This is not a family that mostly relies on nannies," the source said. "Gwen and Gavin are great parents and mostly handle school drop-offs and pick-ups themselves. Gavin is a jokester and often has the boys doubled over with laughter and Gwen is very nurturing. She often shows up to afternoon business meetings with Apollo napping in a baby carrier."

For the small smattering of insiders who knew trouble was brewing, other sources close to the couple seemed shell-shocked.

Us quotes a friend of Gwen's who said the No Doubt frontwoman "seemed very happy and was gushing about her life with four boys." The insider added that there were "no signs" a split was imminent.

In February, Gwen even talked positively about her marriage in an interview with Cosmopolitan Magazine. "We go through so much together -- it's a miracle that we could stay together this long. [Gavin's] on a short tour right now, and he wrote me the sweetest note this morning," she said. "It's good to have those days when we both do our own things. I think that's what keeps relationships going, when both people can be themselves and have their own individuality."