Kristin Cavallari and Jay Cutler 'Were Not Suited for Each Other' from the Start, Source Says

Kristin Cavallari and Jay Cutler have split after a decade together, and according to sources, they've had issues for a while.

"Kristin has been so unhappy for so long. Her marriage has been struggling for at least three years," an insider close to Cavallari tells PEOPLE. “She just wants to be happy and to be a good mom. They tried to make it work for the kids, but Kristin realized Jay is not going to change.”

A TV entertainment source tells PEOPLE that the reality star turned entrepreneur and former NFL player "were not suited for each other, even in the beginning."

"They were both so busy with their careers that it was often hard to find time to be together," the source says. "They lived apart a couple of times during the marriage. This takes a toll on any romance no matter how promising things looked to others."

The Hills and Laguna Beach alumna started dating the Chicago Bears quarterback in 2010. They got engaged in 2011, then split briefly that year only to reunite just weeks later.

After Cutler retired from the NFL, they moved outside of Nashville, Tennessee, where they've been raising sons Camden, 7, Jaxon, 5, and daughter Saylor, 4. On Sunday, they announced their split with a joint statement on Instagram, saying it was just a situation "of two people growing apart" and that they had "nothing but love and respect for one another."

According to the entertainment source, Cutler and Cavallari's relationship got even more complicated after they had kids together.

"Each is a good parent, but they seem to do better jobs with the children when they parent them separately," the source says. "They have too many disagreements over style and what is right or wrong when they're together."

"Like any couple, they want to stay together for the kids," the source adds. "But once the spell is broken and the romance has fizzled out, it's just a waiting game to see how long you can stay together until an explosion occurs. That is where they were."

Another industry source also tells PEOPLE the split was a "long time coming" as the two "simply were not getting along."

Reps for the stars have not responded to PEOPLE's requests for comment.

Cutler, 36, cited "irreconcilable differences" as the grounds of separation in his divorce filing, while Cavallari, 33, cited both "irreconcilable differences" and "inappropriate marital conduct" in her counter-complaint. They disagree on when they formally split; she listed their date of separation as April 7, the day they returned back to the U.S. after spending several weeks in the Bahamas amid the coronavirus pandemic, and he listed April 21, the day he filed the paperwork.

In her filing, Cavallari submitted that "any misconduct alleged or implied against her" in Cutler's divorce petition "was in response to and/or as a result of misconduct on [Cutler's] behalf," though Cutler did not allege misconduct against Cavallari in his filing.

Cavallari is seeking primary physical custody of their kids with "reasonable periods of parenting time" for Cutler. Cutler is seeking joint custody.

"Kristin’s number one priority will always be her kids. She’s done everything for them," the Cavallari insider says. "While Jay was playing, she was basically a single mom for eight years. Jay claiming he’s the primary caretaker is completely untrue. She wants full custody, but she has no intention of taking the kids away from their dad. She wants him to have visitation."