Gwen Stefani and Gavin Rossdale used to be together, every day together, always. But now it appears, Stefani could be losing her best friend after news broke Monday night that the couple filed for divorce after 13 years of marriage.

Their three kids, 9-year-old Kingston, 7-year-old Zuma, and 17-month-old Apollo are going to be who both focus on throughout the divorce, People reports. The No Doubt frontwoman wants the kids to feel comforted and secure through the process.

“Their priority is making sure they maintain a safe and secure home for their children regardless of what's happening,” a source told People. Both Rossdale and Stefani have applied for joint custody.

The couple released a statement to E!, saying, “While the two of us have come to the mutual decision that we will no longer be partners in marriage, we remain partners in parenthood and are committed to jointly raising our three sons in a happy and healthy environment.”

Stefani and Rossdale married in 2002 after meeting while their bands were on tour in 1995. The split is reported to be an amicable one between the two.

In March, Stefani told Cosmopolitan that it was “a miracle,” the couple had stayed together as long as they have. “[Gavin's] on a short tour right now, and he wrote me the sweetest note this morning. 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,” she dished.

Stefani has made a name for herself through the success of No Doubt and her solo career, which included hit songs like “The Sweet Escape” and “Hollaback Girl.” Recently she has been featured as a coach on NBC's “The Voice.”

Rossdale is the lead singer and rhythm guitarist of Bush, who is known for their hit 1994 album “Sixteen Stone,” and 1996's “Razorblade Suitcase.” They've released hit songs like "Machinehead," "Little Things," "Glycerine," and "Everything Zen." The band disbanded in 2002 but reunited in 2010 and have been active ever since.