"Kourtney really seems finished with Scott this time," a source tells PEOPLE.
As for the former couple's custody agreement for their three children (Mason, 5, Penelope, 3, and Reign, 9 months) the source says "everything is getting finalized," though "there is nothing in writing yet."
"However, Kourtney won't allow Scott split custody," the source adds. "She thinks he is a drunk mess and not fit to be a parent without supervision."
Disick has not responded to requests for comment. His substance abuse has been chronicled on Keeping Up with the Kardashians for seasons, and he's had a few stints in rehab throughout their nine-year relationship.
Kardashian, 36, will "only agree to supervised and limits visits" between Disick and their children, the source adds.
The two are "also working out a financial deal," the source continues.
As for Kardashian, the reality star "is doing really well."
Though she "keeps an eye on Scott via social media," she has "come a long way and instead of getting upset about Scott's latest shenanigans, it helps her realize splitting was for the best," the source says.
Most recently, Disick, 32, was spotted with 18-year-old model Lindsay Vrckovnik, partying in New York City and hitting the beach in Miami.
The source says Kardashian "knows he was hanging out" with Vrckovnik and is making her kids her priority.
"Although being a single mom is not easy, Kourtney is used to taking care of the kids by herself," the source says. "She has them on a great schedule and everything runs smoothly even when she is alone with them. She had some nanny help, too, since she is working, but she spends as much time as possible with the kids."
The three youngsters "are her world and she loves being a mom," the source adds.
Kardashian called it quits with Disick after he was photographed cozying up to another woman while on vacation in Monte Carlo this summer. Their emotional breakup was recently chronicled on the season 10 finale of Keeping Up with the Kardashians.
"Kourtney both looks and seems much happier," the source says. "She seems much more relaxed, too."
It seems Disick agrees: Last week, he took to Instagram to compliment his ex, posting her recently released nude Vanity Fair photo and writing, "Now that's 1 hot mama."
Though a source told PEOPLE last week that Disick "feels free and ready to have fun," the reality star is sending a different message.
On Monday, Disick took to Instagram to post a captionless photo of the words "Never Give Up."
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Kardashian has also posted several cryptic messages to her social media over the summer, and most recently tweeted a heartfelt thank-you to her fans after the photos of Disick and Vrckovnik surfaced.