"It was not an easy decision, obviously," Cavallari admits. "It was something that I truly thought about every single day for over two years."
After 10 years together, the couple, who are parents to 8-year-old Camden, 6-year-old Jaxon and 4-year-old Saylor, filed for divorce in April, with Cavallari citing "marital misconduct" and "irreconcilable differences" as the reason for the breakup.
"It was the hardest decision that I have ever made," she tells ET of her split. "But I don't know, my mom [Judith Spies Eifrig] used to say, 'You'll know when it's time.' I feel like that was true. I knew. And that's that."
Cavallari and Cutler, who were married in 2013, announced their breakup in a joint statement on Instagram less than a month after they returned from a three-week "isolation" in the Bahamas with their kids and two friends.
"With great sadness, after 10 years together we have come to a loving conclusion to get a divorce. We have nothing but love and respect for one another and are deeply grateful for the years shared, memories made, and the children we are so proud of," their statement read. "This is just the situation of two people growing apart. We ask everyone to respect our privacy as we navigate this difficult time within our family."
Tune into Entertainment Tonight on Monday, Sept. 28, for Cavallari's exclusive, first post-split, in-person interview, where she goes in-depth about co-parenting with Cutler and how they're both coping following their breakup.
Cavallari fans can also check out her new cookbook, True Comfort: More Than 100 Cozy Recipes Free of Gluten and Refined Sugar, when it hits shelves on Sept. 29.