Palin refers to Johnston as a "gnat" throughout the book and claims he "cheated on me about as frequently as he sharpened his hockey skates." She writes that while her parents accepted her pregnancy, he wasn't so supportive, saying the child, "Better be a f**king boy."
Palin told "GMA" that even though her parents, Sarah and father Todd, were supportive of her pregnancy, the hardest part of the whole experience was the day she had to return home after Johnston told her he might have impregnated another girl, just as news of their engagement was splashed on the cover of US Weekly magazine.
"It was the longest drive of my life, driving down that driveway," she said. "It was the hardest thing to I've ever had to do."
But her family rallied by her side, which Palin told "GMA," made the experience worth the negativity and criticisms she faced.
"Having my family on my side, that's all that matters," she said.
The "tough skin" Palin told "GMA" that she shares with her well-known mom has helped her as she has become a celebrity in her own right, through her broken relationship with Johnston, her time as a contestant on "Dancing With the Stars" and, most recently, a barrage of negative media after undergoing what she labeled "corrective surgery" on her jaw.
"Whatever they say, it really doesn't bother me," Palin said of her reaction to critics. "Especially something like that."
Up next for Palin is an even bigger share of the Hollywood spotlight. She's now working on a reality-TV show of her own with her former "Dancing with the Stars" contestant, Kyle Massey.
In the book, however, Palin writes of not-so-happy memories from her 2010 stint on the show, where she and her professional partner, Mark Ballas, finished in third place.
"I noticed some of the contestants rolled their eyes when they realized we'd survived to dance another day," she wrote, adding that some of the contenders voted off "literally would not speak to us or acknowledge our presence."
"I knew that I needed to do something to provide for Tripp," she told "GMA" of her decision to put herself in the reality-TV spotlight. "I knew that we would financially gain from it."
Something not in the future for Palin, however, is following her mom into a political career, she said.
"We're both very independent," Palin said of the traits she shares with her mother. "We're both strong.
"But, no," she said of politics, "not on the top of my list."
ABC News's Sheila Marikar contributed to this story.