Kunis didn't go into detail about what happened between the two but seemed to imply that their split was her fault.
"When I got to be single I said, 'I just need to figure myself out,'" she recalled. "'I genuinely need to know why I did what I did and, like, regroup myself as a human being.'"
When Shepherd asked Kunis if she has forgiven herself for what happened, she seemed to respond in the affirmative.
“You know, it’s been so long like I feel like enough time has gone by,” she said. “I think enough mutual friends have been like, everybody kind of forgave everybody for what went down.”
Both Kunis and Culkin have now moved on — Culkin is dating actress Brenda Song and Kunis is married to Ashton Kutcher, with whom she has two children, daughter Wyatt and son Dimitri.
Kunis and Kutcher met on the set of That '70s Show when they were young, but didn't start dating until one year after Kutcher announced his divorce from Demi Moore.
On Shepherd's podcast, Kunis revealed the timing of their relationship caused a bit of reservation from her mom, who initially didn't seem thrilled to hear about her daughter's new relationship.
"We were driving up to Laurel Canyon, and I was like: 'Mom, I have to tell you I'm dating somebody,'" Kunis recalled. "And she was like: 'Oh, tell me! Who are you dating?' And I was like: 'You've got to brace yourself for this one. I'm dating Ashton Kutcher.' And she literally was like: 'Shut the f— up' in Russian."
The 34-year-old explained that her mom's reaction was largely due to "misconceptions" about Kutcher's split from Moore.
"He was coming off of a divorce. He was post, post, post, way post-divorce but I think there was a lot of misconceptions about the divorce," Kunis said. "We were crazy neurotic about it keeping it a secret, probably to a massive fault where we alienated so many friends, I'm sure."
Photo Credit: Getty / Chris Polk