Simpson, 39, spoke to Hoda Kotb in an exclusive interview on TODAY Wednesday about a dark part of her childhood that she also addresses in her new memoir, "Open Book," which will be released on Feb. 4.
Simpson writes that she was sexually abused when she was 6 years old by a young girl who was a family friend.
"At the time I didn't really understand what was happening,'' she said. "I knew something was wrong. I knew it was wrong what was going on. This was a very close person, and she was being abused. It happened throughout a long time in my life."
She struggled to tell her parents, particularly her father, who was a Baptist youth minister in Texas.
"I was a preacher's daughter,'' she said. "I was taught to be a virgin until I got married, and so I never wanted to share these sexual things that were happening because I didn't want to hurt anybody."
Simpson eventually summoned the courage to tell her parents, who put a stop to the abuse.
"My parents' reaction — they did the best that they could,'' she said. "That's a heavy thing to hear from your child.
"They ignored it with their words, for sure, but they took action, and I never had to do the sleepovers again. I never had to go back."
Simpson also spoke about her struggles with alcohol, the deterioration of her first marriage to Nick Lachey, and her struggles with body image that began when she became a pop star as a teen.
"People are really going to understand me, I think, for the first time,'' Simpson said about her new book. "They're going to see everything that I have gone through and all the fear and the struggles that I did face and how I came through on the other side."
She addressed her issues with alcohol that surfaced in 2017 after she had found love with her second husband, retired NFL player Eric Johnson, and had two children.
Simpson's appearance on an episode of "The Ellen DeGeneres Show" had fans wondering if she had been drinking.
"I can't even watch the interview,'' she said. "It was a weak moment for me, and I wasn't in the right place. I had started a spiral, and I couldn't catch up with myself and that was with alcohol."
Her drinking was constant, but eventually there was an incident that prompted her to get help. She started drinking at 7 a.m. on Halloween in 2017 and got so inebriated she couldn't take care of her children.
"I honestly couldn't even tell you who got them ready,'' she said. "I was just dazed and confused, and I just wanted to go to sleep. I didn't take them trick-or-treating. I didn't show up for my family. I took the picture and I made the world think that I showed up."
The next day she quit drinking and began working with a therapist.
"I just realized that I had to surrender,'' she said. "I just want to continue on the path that I'm on, and at this point in my life, now I'm strong enough to deal with anything that comes my way. Because I don't have something to retreat to that will numb me from actually going through it."
Simpson's issues with alcohol came years after she began struggles with her body image. She signed to a record label as a teen and said she was told to lose 15 pounds, which started a spiral that lasted decades.
"I had to go on a crash diet, quickly,'' she said. "I took diet pills. I didn't know what I was doing. I didn't know how much I was taking. It ended up being something I did for 20 years."
She also got married at 22 to Nick Lachey of the boy band 98 Degrees despite her father cautioning her that she was too young to tie the knot.
The couple became reality television stars with the MTV show "Newlyweds: Nick and Jessica" from 2003 to 2005, but Simpson admits that the success also contributed to the end of their marriage.
"I would watch episodes back and I didn't like what I saw,'' she said. "I saw what people were laughing at and what was loving about it, but I also saw a lot of eye-rolling, and that wasn't just editing."
She specifically mentioned an incident in which she was given the cover of Rolling Stone magazine without Lachey.
"He was supposed to be on the cover with me, and they were like, 'no,' " Simpson recalled. "I felt a lot of resentment. I felt that the love wasn't enough. I'm not a liar. I cannot stay in something and play a role. I've tried it many times in my life and I just can't do it."
Now she has found love with Johnson, with the two celebrating the birth of their third child, Birdie Mae Johnson, in March 2019. Simpson is also releasing her first new music in 10 years with a deeply personal set of songs to go with her memoir.
"It's about walking through fear and it being okay to be afraid,'' she said. "And the other side of fear is what's so beautiful. That's when you get the reward."