While she doesn't specify the abuser, Lopez writes, "I've never gotten a black eye or a busted lip, but I've felt abused in one way or another: mentally, emotionally, verbally." (She does not claim any physical abuse.)
Her new book describes the many ups and downs of her love life, as well as the reasons behind her 2011 split from Marc Anthony, the father of their 6-year-old twins, Max and Emme.
But this wasn't the book she planned to write.
"It started out as fan book, a diary of the years I went on tour," Lopez tells PEOPLE in this week's issue. "But it turned into something else. As I started looking at things, it helped me face a lot. I would never go into specifics about my relationships, and I don't. But the idea was that I learned something."
"Every day that you don't walk out that door, every day you accept things in your partner and in yourself, is a day that you're saying it's okay," she writes.
"I had to learn to take care of myself and know my worth," the American Idol judge also tells PEOPLE. "When things are not working out, and you're not being treated in a way that you want to be treated, you have to look at that because it's nobody's fault but yours. You have to take control and you have to set up your own boundaries. You have the power to change it."
Anthony, she says, is supportive of her and her new book. "He read it and said, 'Wow, I didn't realize you were going through this whole journey,' " says Lopez. "We try to be supportive of each other. We feel that's the best thing for our kids."
For Lopez, who broke up with choreographer Casper Smart in June, it's now about putting all these lessons into practice.
"I'm learning to set up boundaries, you know what I'm saying?" says the singer, who adds that she is definitely single. "I call myself a W.I.P. – a work in progress. I'm still learning and growing every single day."
For more on Jennifer Lopez, including an exclusive excerpt from her new book, pick up this week's issue of PEOPLE, on newsstands Friday