"Yes, I have filed for divorce," the actress, 45, told People magazine, in an interview that will appear on newsstands Friday. "I am sad and I am scared."
The Oscar-winning "Blind Side" actress has been married to motorcycle mogul James, 41, for five years. They separated last month after reports that he cheated on her with multiple women.
Bullock also revealed that she has adopted a son, 3½-month-old Louis, born in New Orleans. She and James began the adoption process four years ago but decided to keep the details under wraps until after the Oscars. Now, she plans to raise him as a single mom.
"It's like he's always been a part of our lives," Bullock told People.
"She has been hiding with the baby since the scandal broke," People magazine senior editor David Caplan said. "She said that when she found out about Jesse's affairs, she grabbed Louis, she grabbed their stuff, and she left."
No one but Bullock's inner circle knew about Louis -- she told People that a friend even arranged for a moyle to perform a bris at her home so that they wouldn't attract attention at a hospital.
In her interview with the magazine, Bullock went into depth about her reasons for adopting. She said spending time in New Orleans following 2005's Hurricane Katrina made her want to adopt a child from there.
"One day it was just a feeling that came that instead of bringing another life into this world, we wanted to reach out and find our child in New Orleans. There were so many children alone after Katrina, and for some reason we knew that someone would want to come into our lives."
"Whether it was a girl or boy, what background -- it didn't matter," she added. "We somehow knew the right little person would come."
Bullock said she didn't receive any special treatment in the adoption process.
"We began the conversations and then all the paperwork, the background checks, the home visitations about four years ago," she told People. "We didn't want to go at it any other way than the way everyone else would have to do it. There are hoops, but the hoops are there to protect the child. And worth every jump."
Beyond that, however, Bullock won't divulge details about how Louie became her son.
"She won't speak to how she physically got the baby or when or the identity of the birth mother," Caplan said.
Meanwhile, it seems everyone wants to say they're sorry to the actress.
James, who has publicly apologized to Bullock and his three kids, recently returned home after a 30-day stay in a treatment facility where he sought help for "personal issues." He told People magazine that he regrets losing Bullock and Louis.
"The love I have for Louis cannot be put to words," James said. "Not having him around to love and to hold has left a huge hole in my heart."
"Sandy is the love of my life, but considering the pain and devastation I have caused her, it would be selfish to not let her go," he added. "Right now it is time for me to beat this addiction that has taken two of the things I love the most in life."
Melissa Smith, one of the women who reportedly had an affair with James, faxed a letter of apology to Bullock Sunday, according to TMZ.com. Her apology followed that of Michelle "Bombshell" McGee, the first woman to claim she slept with James.
"I felt I needed to reach out to you and tell you how deeply sorry I am for everything that you've been through," Smith wrote. "I am sorry for any hurt or pain that I have caused you. My actions of engaging with a married man are unforgivable. I never meant you any harm."
Smith added, "I compromised my beliefs on several occasions and as a result will never forgive myself," before concluding, "Please contact me if you wish to discuss on the phone or in person."
Piror to today's divorce announcement, Bullock had only addressed the reports about James' alleged cheating once. At the beginning of April, she broke her silence -- not to discuss her marriage but to deny rumors of the existence of a sex tape.