"Kevin and Christine have split several times before. She has often given him ultimatums and threatened to end their relationship if he didn’t comply," a source spilled to a news publication.
And while Baumgartner, 49, tried to put the blame of their marital demise on Costner, 68, friends of the couple, as well as his older children from his first marriage, "have taken his side in this," the insider noted.
Costner is the father of Annie, 39, Lily, 36, and Joe, 35, whom he shares with his ex-wife, Cindy Silva, 66. The former flames tied the knot in 1978 and finalized their divorce in 1994 after rumors swirled that the Bodyguard star had been unfaithful to his wife.
The award-winning actor welcomed his son Liam, 27, with Bridget Rooney in 1996. The pair had a brief relationship together at the time.
Costner later married Baumgartner in 2004. The separated spouses share Cayden, 15, Hayes, 14, and Grace, 13.
"The divorce is hitting the younger kids hard because obviously they do spend more time with their mom," the source pointed out of Costner and Baumgartner's teenagers.
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As the exes duke out their nasty divorce in court, as well as custody of their three kids, Baumgartner has refused to vacate Costner's $145 million mansion, causing another bitter situation to be hashed out between the pair's lawyers.
Costner filed a petition last month asking a judge to demand Baumgartner leave his home, citing their prenuptial agreement that instructed his estranged wife to move out of the property within 30 days of filing for divorce.
Though she hasn't physically left the residence, in regard to their relationship, "Kevin thinks Christine abandoned him," the confidante noted, insisting Baumgartner "didn’t give him the opportunity to fix their marriage before calling it quits."
"He really was hoping to work things out to avoid this mess. But instead, it’s only getting uglier," the source concluded.
In Touch spoke to a source about Costner and Baumgartner's split.