In a snippet obtained by Entertainment Tonight, the star reflected on the end of her relationship with Lachey, now 47. The two were married from 2002 to 2006.
Less than a year after the split, the 98 Degrees alum began dating Vanessa Minnillo -- now Lachey -- who he married in 2011.
"So, Nick, you're with another already? Seems that you forgot the love you spoke to me," Simpson said in the handwritten diary passage. "I'm saddened beyond belief."
She added: "Alone in the dark, with no one to call my own."
Reps for Lachey did not immediately respond to Fox News' request for comment, while reps for Simpson declined to comment further.
Simpson previously opened up about her relationship with the singer and revealed one of the reasons behind their divorce.
"With Nick, I was 22 years old when we got married... Once I started to get endorsement deals that were separate from him, there was a little bit of tension," she said. "[The press] put us as a trophy couple that we actually weren't."
These days, Simpson said she's "happy for" her ex.
"We both were at fault in a lot of things, but I'm happy for him now. He's married with three beautiful children," she explained. "He took heartbreak and made it into something beautiful."
Lachey and his wife share three children: Camden, 8, Brooklyn, 6, and Phoenix, 4.
"That's what I did as well, but it took me longer to get there," Simpson added.
She is now married to former football pro Eric Johnson. They also share three little ones: Maxwell, 8, Ace, 7, and Birdie Mae, 2.