ET was the only entertainment news show to sit down with the 39-year-old singer to talk about her new memoir, Open Book -- which hit shelves on Tuesday -- that includes insight into her marriage to 46-year-old Lachey. Simpson and Lachey got married in October 2002 and their relationship was famously shown on their MTV reality show, Newlyweds: Nick & Jessica, before they announced they were separating in 2005. Their divorce was finalized by June 2006.
During ET's sit-down with Simpson airing on Tuesday, she said that she definitely thought she was going to be with Lachey for life.
"With Nick, I was madly in love," she tells ET's Brooke Anderson. "I mean, I loved everything about the guy and I wanted to spend the rest of my life with him."
She also noted that her dad and former manager, Joe Simpson, didn't approve of them marrying because she was only 22 at the time.
"I was, you know, 22 years old when we got married but I thought I knew my future and I thought he was gonna be in it forever," she says. "I thought my dad was completely wrong. I was like, 'No, I got this figured out. Watch.' You know? Like, I'll prove you wrong."
Meanwhile, Open Book -- which is guided by Simpson's journals that she's kept since she was 15 -- is out now. In addition, the Open Book Audiobook includes six exclusive new songs inspired by her intensely personal memoir.
For more on Simpson's book, watch the video below: