The actress - who announced the hiatus last year - will return in Extremely Loud and Incredibly Close alongside Tom Hanks.
"It's true, I didn't want to go back," Bullock told Entertainment Tonight.
She said that Stephen Daldry's movie - in which she plays the wife of Hanks's 9/11 victim - was an opportunity she could not turn down.
"It was an opportunity as an actor that you hope you have and you're given it, as long as it was a great experience for me and my son at the same time, then it was an experience I wanted to take," she said.
Bullock took the hiatus following her divorce from husband Jesse James, her 'Best Actress' Oscar win for her role in The Blind Side, and the adoption of her first child.
Daldry has said that Bullock "was very keen to get back to work".
Extremely Loud and Incredibly Close will receive a limited release on Christmas Day followed by a general release in the US on January 20 and in the UK on February 3.
Watch the Entertainment Tonight interview clip with Sandra Bullock below: