"I had a horrible breakup. I had a horrible, horrible, horrible breakup," she said about Culkin.
"I f--ked up. I was an a--hole in my 20s and I'll be the first to admit it," the actress continued. "And that's kinda something that took me a long time to come out and be like, 'Yeah, you know what, I was a d--k,' and accept it and I own it."
"It's f--ked up what I did. It's f--ked up what I did, and it's f--ked up how I did it," she continued, unfortunately not elaborating on what her big mistake was.
Kunis went on to say it was "super hard" for her to forgive herself. "It's been so long, I feel like enough time's gone by, I think enough mutual friends have been like everybody kind of forgave everybody for what went down."
Since she wasn't getting into the specifics, Shepard joked, "People don't know this but you killed one of Macaulay's brothers."
"It's been however many years now, I think everybody's in a much better place," she said.
Kunis would be single for two years before she started hooking up with her now husband Ashton Kutcher ... but she was "open for business" during the interim.
"When I got to be single I said, I just need to figure myself out. I genuinely need to know why I did what I did and, like, regroup myself as a human," she said. "So I just did that and, subsequently, after 6pm was open for business."
"I didn't want to date, I didn't want to be taken out to the movies," she said, before adding that her Kutcher hookup was the first time she "spent the night at somebody's house" in those two years.
The two would tie the knot in 2015 and are now parents to Wyatt and Dimitri.