In the actress' new book, "Gracefully You: How to Live Your Best Life Every Day," she revealed that she "felt blindsided' when she learned about Tatum's relationship with the British singer.
Dewan, 38, and Tatum, 39, announced their split in April 2018. In October that year, Tatum and Jessie J entered a high-profile relationship.
“I was learning things about my ex most people wouldn’t have to face -- and over the internet, as it was happening,” Dewan wrote in her book. E! News published an excerpt of the book on Thursday.
Dewan said she was alone on a plane when she heard the news, and that "choosing grace as I learned everything about my personal situation along with the rest of the world was really difficult."
In the excerpt, Dewan also discussed the days following her split, which occurred the night before she began filming the second season of "World of Dance."
"In the beginning of my separation, it felt as though I were in a dark closet, desperately trying to find the light or the way out," she said. "I was in a state of shock. One week I'd be doing really well and the next I was slammed with a whole new slew of emotions. The rumor mill was churning out story after story. There were many times I hid under the covers, wondering what was next. The pain hit me like a tumbling avalanche. I was completely overcome with fear and sadness. It took many moments of sitting alone with my grief to force me into surrendering to my roller coaster of a situation."
In November 2018, Jessie J, 31, took to Instagram to slam commenters that compared her appearance to that of Dewan.
Shortly after, Dewan tweeted her support for Jessie J's statement and said it was a "beautiful message."
Tatum isn't the only one to find happiness after the split, as Dewan is now expecting a child with her boyfriend Steve Kazee.
“When I was ready, I started dating someone amazing,” she wrote in her book. “It was this cosmically great thing where we circled back around each other after a moment of instant recognition years ago.”
It was also recently announced that Dewan would be hosting a new dating-dance competition hybrid television show called "Flirty Dancing." The show will teach partial dance routines to participants, who will then meet their partners to perform the routine in its entirety.
Dewan’s book hits the shelves on Tuesday.