"I had a horrible breakup. Horrible, horrible, horrible breakup," Kunis told host Dax Shepard.
"I was an [expletive] in my 20s, and I'll be the first to admit it," she said. "And that's something that took me a long time to come out and be like, 'Yeah, you know what, I ... accept it and I own it now.'"
Kunis didn't specify how the split happened, but regrets how she ended the relationship.
"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 being,'" the star recalled.
Kunis and Culkin split in 2011 after eight years of dating. Kunis has since married her former That '70s Show co-star Ashton Kutcher, while Culkin is dating former The Suite Life on Deck star Brenda Song.
"She just does nice things, and I feel like at first I didn't know how to be treated well in that kind of way. It was a strange, almost foreign concept. So that kind of thing ... I realize it is important to me," he added.
Kunis and Kutcher married in July 2015. The couple are parents to 3-year-old daughter Wyatt Isabelle and 18-month-old son Dimitri Portwood.