Jessica Simpson Was ‘Saddened Beyond Belief' After Nick Lachey Moved on With Vanessa

Jessica Simpson is looking back on her divorce from Nick Lachey in new entries to her memoir, "Open Book."

In an excerpt shared by Entertainment Tonight, the "Dukes of Hazzard" actress reveals how heartbreaking it was to learn of her ex-husband's relationship with Vanessa Lachey, formerly Minnillo, just months after they went their separate ways.

"So, Nick, you're with another already? Seems that you forgot the love you spoke to me... I'm saddened beyond belief," she said. "Alone in the dark, with no one to call my own."

In the memoir, released last February, Simpson detailed how their marriage of four years publicly unraveled as they starred on the MTV series "Newlyweds: Nick & Jessica."

Though many assumed the reality show was the cause of their divorce, Simpson said that wasn't the case.

"I don't believe that the show is what tore our marriage apart," she said.

Jessica Simpson's Family Album

"Nick and I were very great at being together publicly and on-camera. We were best at our relationship when we were singing together. I felt like we were at home in that place," she continued. "But as far as doing the reality show, we just kind of had fun with it until the end – until we started having marital problems."

In addition, she revealed in the memoir that she had an emotional relationship with "Dukes of Hazzard" co-star Johnny Knoxville, which she felt was "worse than a physical one."

The star, who famously saved herself for marriage, wrote in the memoir, "After I actually had sex, I understood that the emotional part was what mattered. And Johnny and I had that, which seemed far more of a betrayal to my marriage than sex."

It wasn't long after the movie premiered that she and Lachey went their separate ways, with the 98 Degrees singer taking half of Simpson's income in the divorce settlement.

Simpson doesn't necessarily regret the marriage, but she feels remorse about never signing a prenup. She reiterated this in an interview with Dr. Oz, explaining how she was "offended" when Lachey first brought it up.

"I was like, 'We're going to be together for the rest of our lives. We're saying our vows to God and in front of all of our family and friends. Like, this never gonna end.' And, we didn't sign a prenup."

What's more, Simpson and Lachey hooked up some months after finalizing their divorce. According to Simpson, she watched a documentary about the making of Lachey's album "What's Left of Me," as well as his "Making the Video" special on MTV for his song of the same title, and decided to invite him over to talk, because she felt like she needed to "fix him." One thing led to another and they slept together.

Coincidentally, Vanessa Minnillo and Nick Lachey first crossed paths when she starred in the "What's Left of Me" video. Five years later, they tied the knot and have since welcomed three children together.

To learn more about the revelations from "Open Book," click here.