A source tells PEOPLE Disick was in "a very dark place" leading up to the split from Kardashian, his partner of nine years and the mother of his three children

By Tara Fowler and Aili Nahas
Updated July 06, 2015 03:45 PM
Denise Truscello/Wireimage

After years of Scott’s troubled behavior, Kourtney put her children first and decided that enough is enough. Subscribe now to get the exclusive inside scoop on how Scott ‘threw it all away’ and why Kourtney finally ended it, only in PEOPLE!

Kourtney Kardashian was fed up with longtime boyfriend Scott Disick‘s behavior, a source tells PEOPLE.

The couple, who have three children, recently split after nine years together – and the source says 32-year-old Disick’s substance abuse issues are part of the blame.

“Scott has a serious drinking problem,” the source tells PEOPLE. “He doesn’t know when to stop, and he takes it too far.”

As viewers of Keeping Up with the Kardasians have seen, Disick’s struggles with moderation led him to check into rehab earlier this year.

According to the source, Disick’s loss of his parents in late 2013 and early 2014 was especially difficult: “His parents’ deaths really hurt him. He took it incredibly hard and was in a very dark place after it happened. It’s still painful.”

RELATED VIDEO: Kourtney Kardashian and Scott Disick Have Ended Their Nine-Year Relationship

Most recently, Disick was spotted cozying up to a woman with whom he was reportedly once involved during a vacation in Monte Carlo while Kardashian stayed at home with the children – Mason, 5½, Penelope, 3 this week, and 6-month-old Reign. He was photographed holding a beer and getting particularly close with Chloe Bartoli, a 25-year-old stylist based out of Los Angeles.

The source adds that Kardashian, 36, was embarrassed by Disick’s recent behavior even before the photo surfaced – and that his out-of-control lifestyle contributed to the split.

“He acts out, he rebels, he drinks too much and he embarrasses her,” the source says. “He acts like a kid sometimes. He just rebels against the crazy responsibility of having three kids and makes Kourtney do it all.”