KELLY Clarkson and Brandon Blackstock 'worked so hard' it was 'impossible to enjoy each other' before the singer filed for divorce.
Their marriage also strained under the pressures of living in Hollywood.
A source told PEOPLE that work really put a strain on the couple's relationship as they prioritized it over their family.
The source said: "Things had changed in their marriage and the split was a while coming.
"They both work so hard. It's nearly impossible to enjoy each other, especially with the children needing constant attention."
The source further added that the couple had been like friends for a while now.
Another source further explained that the couple's split took most of their friends by surprise.
"It just came out of nowhere. Their marriage was so strong," says the source close to the former couple.
However, Kelly had previously expressed that they were really struggling while quarantined in Montana.
"We've been in really close quarters and it's been kind of nuts. I'm not going to lie," The Voice coach told PEOPLE.
"There's definitely some cabin fever going on. It's challenging being a working mom."
Now in Los Angeles, Kelly has been focusing on her career, her healing, and her children.
"Kelly's been through a lot, and she thought Brandon was her happily ever after," the source said.
"It's very sad."
A source had previously revealed that it was actually Kelly's career was the one taking up most of the time as she was working on her next album, coaching for The Voice, and filming for her daily talk show.
An insider told Us Weekly that when she went to Hollywood, it changed things," adding that Brandon "prefers their quiet life in Nashville."
Kelly - who shares daughter River, six, and son Remington, four, with Brandon - filed for divorce on June 4, citing "irreconcilable differences."
Delving into the reason for their split, a source said: “They clashed on so many levels, and being in quarantine together heightened their problems to the point of no return so she filed for divorce.
“Everyone thought Kelly and Brandon had the perfect marriage, but they definitely didn’t. Brandon’s very laid-back, whereas Kelly’s pretty high-strung.”
The source claimed Kelly's career moves in recent years put pressure on their marriage.
The Since U Been Gone singer joined The Voice as a coach in 2017 and started her own talk show, The Kelly Clarkson Show, in September last year.
Kelly's been open about how she's doing since filing for divorce, recently admitted to having "abandonment issues" after their split.
Meanwhile, sources close to Kelly and talent manager Brandon, 43, previously told Entertainment Tonight that quarantine made things worse in the couple.
The insider - who said that those closest to them knew things were bad - claimed: "Kelly and Brandon had been having problems for several months and were making a conscious effort to work things out.
"They both hoped quarantining away from LA in Montana would help them work things out in their marriage, but instead the change in environment was actually detrimental."
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Just last month, Kelly put her $10 million LA mansion on the market while she and her family were quarantining in Montana.
However, she took it back off the market weeks later.
Early in the coronavirus pandemic, Kelly and Brandon were isolating at the Montana mansion they picked up a year and a half ago named Vintage Valley.