"I blew it because I didn't think I was good enough for her," he told "Nightline." "She has one way she wanted to live her life and I had another way to live mine and I was just too insecure.
"I did feel like the Phantom of the Opera hiding in the catacombs," he added. "People have actually said that I didn't deserve to be with her."
Watch ABC's Juju Chang's full interview with Billy Bob Thornton on "Nightline" tonight at 11:35 p.m. ET/PT.
In his new book, "The Billy Bob Tapes: A Cave Full of Ghosts," which was recorded and then transcribed, Thornton, 56, talks about growing up in Arkansas and eating squirrel, the real-life colorful characters that wormed their way into his screenplays and music, and how this creative genius struggles with ADD and dyslexia.
“I have a very hard time reading,” he told “Nightline.” “It’s hard to explain to somebody how I read. I can’t read normally. I can’t pick up a book and read left to right and get something out of each sentence.”
A country singer turned actor, Thornton has taken on his movie roles with stark serious. He would gain and lose weight for roles, and even lived off a can of tuna and Twizzlers for months so he could look “gaunt” for a particular film.
“If you want to [lose weight] quickly, you just pick something and because I happen to know if you eat a can of tuna and a pack of Twizzlers, you will lose weight, I promise you,” he said.
Thornton also talks about relationships built and lost in the book. Thornton has been married five times, but his most famous relationship was with Jolie, whom he starred with in the 1999 film, "Pushing Tin." By the time they were married in 2000, both were Oscar winners -- Thornton for his "Sling Blade" screenplay, Jolie for her supporting role in "Girl, Interrupted" -- but Jolie had started to rise to mega-stardom for her portrayal of video game heroine Lara Croft in "Lara Croft: Tomb Raider" (2001).
"When you're in a relationship, any two celebrities or whatever, you know, I think that puts on a lot of pressure," Thornton said. "When Angie and I got married, during that time, I was more famous than she was to start with and then when she becomes this big thing, it's hard in these relationships."
Their three-year-long marriage, which ended in 2003, was plagued with rumors of wild sex acts and dark love rituals. But Thornton said reports that he and Jolie were "vampires with a dungeon" were "absolutely not true," and even reports that they wore vials of each other's blood around their necks were blown out of proportion.
"[Jolie] bought these clear lockets, they were little bitty things," he said. "Angie and I literally poked each other in the finger with a pin and rubbed a little bit of it on that locket. When we were apart, it was a symbolic thing to do."
Nine years later, Jolie penned the forward to her ex-husband's book, in which she writes warmly about Thornton's quirks and funny times they shared together -- including a surprise party gone awry. At one point, Jolie calls him the "mad mathematician" and writes, "To him, I am the number four. May sound strange, but it means a lot to me."
When asked what "number four" refers to, Thornton said, "That's something that no one will ever know."
Jolie was Thornton's fifth wife, but he has long suffered from OCD.
"I have a number attached to all of the people I love," Thornton said. "It represents something."
It's clear that Jolie and Thornton remain fond of each other, and Thornton said they remain "good friends" today.
"We had a great time together," he said. "We had a great marriage and I chickened out because I didn't feel good enough. That's all that happened. It was no big deal, we never hated each other."
"The Billy Bob Tapes: A Cave Full of Ghosts" will be in stores on May 15.
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