The couple's relationship was left in tatters when the 22-year-old actress was photographed cheating on Pattinson with married film director Rupert Sanders.
The "Twilight" co-stars are said to have had a tearful heart-to-heart, with the 26-year-old actor eventually deciding to forgive his girlfriend for what he considers to be a 'stupid mistake', reported Sun online.
"They pretty much decided they couldn't live without each other. Kristen poured her heart out to Robert and told him it was a one-off and a mistake," a source said.
The pair are now said to be moving into a secluded Los Angeles home together and are hoping to rebuild their relationship.
"Rob sees it as Kristen made a really stupid mistake. After a lot of long tearful talks, they've worked it out. Rob can see how truly sorry Kristen is and has totally forgiven her. They really do love each other," the source added.
Stewart seems to have been quietly confident about making amends with Rob for some time.
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Earlier this month, she had said that the pair were "totally fine" about making potentially awkward public appearances together in November while they promote "The Twilight Saga: Breaking Dawn Part 2".
Back in July, Stewart was photographed in the arms of father-of-two Sanders.
In the wake of the news, both were quick to make public statements saying that their tryst had been a mistake.