Billy Bob Thornton says he "blew" his marriage to Angelina Jolie because he did not think he was "good enough" for her.
He and Jolie, 36, met on the set of the 1999 movie "Pushing Tin." They dated for a couple of months a year later and married in 2000. She was his fifth and most recent wife. They made headlines for wearing lockets containing traces of each other's blood. She also had a tattoo of his name on her arm.
She had the ink removed after they split. They broke up three months after she adopted her first child, Maddox, from Cambodia. Their divorce was finalized in 2003.
"I blew it because I didn't think I was good enough for her," Thornton said on "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. People have actually said that I didn't deserve to be with her."
"We had a great time together," he added. "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."
Jolie went on to begin a relationship with Brad Pitt. They currently have six children, including a biological daughter and a twin boy and girl. They announced their engagement in April. Thornton has two sons, Willie and Harry, and two daughters - Amanda and Bella. The latter girl's mother is his current girlfriend, Connie Angland.
Jolie and Thornton remains friends. She even penned the foreword to his book, in which she praises the actor and says he has a "big beautiful heart."
In "The Billy Bob Tapes: A Cave Full of Ghosts," Thornton shares moments from his childhood and adult life as an actor and musician.
The Arkansas native talks about his love for squirrel meat, recalls living for years without running water or electricity, listening to Elvis Presley on the radio when his family had it installed, smoking his first cigarette as a child and burning down a shed and living on a diet of canned tuna and Twizzlers to lose weight for a movie role.
He also reveals that he suffers from dyslexia and obsessive compulsive disorder. Jolie says in the book's foreword that being with Billy Bob Thornton is like "being with a mad mathematician."
"He is constantly counting and repeating," she writes. "To him, I am the number four. May sound strange, but it means a lot to me."
When asked on "Nightline" about what "number four" refers to, Thornton said, "That's something that no one will ever know," adding: "I have a number attached to all of the people I love."
Thornton appears to support ex-wife Angelina Jolie's relationship with now-fiance Brad Pitt.
"She's great for Brad, Brad's great for her," he says in his book. "I'm with Connie, who I love. Angie and Brad are doing great," he says. "I support them 100 percent. I'm right where I need to be and she's right where she needs to be. She and I are still real close, but we keep our friendship to ourselves. It's sacred."