Simpson tells ET she was "madly in love" with Lachey when they got married, even though her father and manager at the time, Joe Simpson, didn't want her to marry him given she was just 22 when they tied the knot.
"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. 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. 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."
Simpson blames the demise of their marriage partly on her skyrocketing fame due to their super successful reality show, and the age difference between the two at the time.
"He's also eight years older than me so he was already so grown up when I wasn't, so once I started to, you know, get covers and get endorsement deals that were separate from him -- 'cause we did everything together -- there was a little bit of tension," she shares. "And, like, a lot less support. And I just felt in our marriage that we really were kind of spinning and trying to tolerate each other towards the end because we were so used to being miced up or onstage or you know, the world took us in so many ways, and they put us as a trophy couple that we actually weren't. I mean, we were at the beginning but it fell out pretty quick."
Simpson alluded to this in an excerpt from her book. "We were young and pioneering our way through reality television, always miced and always on," she writes. "We worked and we were great at it but when it came time to being alone, we weren’t great at it anymore."
"I mean, we just grew apart in so many ways and you know, we both were at fault in a lot of things, but I mean, I'm happy for him now," she reflects. "He's married with three beautiful children and you know, that was his purpose, and he took heartbreak and made it into something beautiful and that's what I did as well. But you know, it took me longer to get there."
Simpson tells ET she would never consider doing a reality show with 40-year-old Johnson.
"I honestly, like, after doing a reality show, I have not been able to watch reality shows," she admits. "Because I know what positions these people are put in. I know when they go out they're forced to drink, I know like, you know, there's so many different things that I just watch happening. I watch the destruction of it and for people to rise above that is a very hard thing to do."
"Been there, done that," she adds.
As for Lachey, he recently made it clear that there are no hard feelings on his part as well when it comes to his failed marriage with Simpson. On Monday, Lachey and 39-year-old Minnillo made an appearance on the Today show and he addressed her candid new book.
"I'll be honest, I obviously haven't read the book, so I don't know what she said or what she revealed there," Lachey said. "But certainly happy for her in her life and I know she is happy for us. There is definitely a mutual respect there."
"Obviously it was a long time ago and we've all moved on," he continued.
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.