Kristin Cavallari Thought Ex-BFF Kelly Henderson Would Reach Out After Jay Cutler Split: 'It's Sad'

Kristin Cavallari has been finding comfort in friends since separating from estranged husband Jay Cutler earlier this year, but her former best friend Kelly Henderson isn't one of them.

"It is really sad," the Uncommon James founder, 33, tells PEOPLE exclusively about the breakdown of their friendship. "I thought that maybe announcing the divorce and everything that [she would reach out], but no."

On the season 3 premiere of Very Cavallari, Cavallari — whose third book, True Comfort, is out on Sept. 29 — revealed she hadn't spoken to the celebrity stylist for two months.

• For more on Kristin Cavallari's life now, including how she's raising three kids and finding new joy after splitting from Jay Cutler, pick up the latest issue of PEOPLE, on newsstands now

"When season 2 was airing, there was a lot of talk on social media about Jay and Kelly having an affair," said Cavallari on the show. "Not for one second have I ever thought that it was true. It wasn't the actual accusations of them having an affair that made me upset, it was how Kelly went about it."

According to Cavallari, Henderson "kept talking about Jay on social media," which was only "adding fuel to the fire." (In May, Henderson denied having an affair with Cutler, telling divorce attorney Laura Wasser on the All's Fair podcast, "I'm just gonna go ahead and put this out there, there was never an affair, I have nothing to do with their divorce, and I am not dating Jay Cutler.")

Now, the cookbook author says she's come to terms with the end of their friendship. "I think that some people are in your life for a certain time period for reasons," she says. "Sometimes we outgrow people, and I actually think that's okay.

"As I get older, I'm kind of like, 'Why hold onto a friendship or a relationship who's really toxic in your life? If it's not bringing you joy, what's the point?' " she continued. "Life is too short. I think it's okay to cut people out of our lives if we need to."

While it doesn't seem Cavallari and Henderson will ever mend their friendship, the former Laguna Beach star says she's been spending more time with her close circle of friends after splitting from Cutler, 37.

"I don't really want to put energy into anything else right now other than my kids, Uncommon James and my friends," says Cavallari, who shares three children — sons Camden, 8, and Jaxon, 6, and a daughter, Saylor, 4 — with her ex. "I'm making time for the people that are really important in my life right now. That's all I care about."