Robert Pattinson fears that Kristen Stewart's affair with married film director Rupert Sanders might have gone on behind his back for months.
According to a friend, the 26-year-old actor is not sure if he can "get over this."
The Twilight actor, who has already quit the Los Angles mansion that he shared with Stewart, wants time alone to decide whether to make their current split permanent.
"He doesn't know if he can move on. He feels humiliated and the wound will take a long time to heal, if at all," the Mirror quoted a source as saying.
The 22-year-old actress has repeatedly insisted that her fling with the 41-year-old Brit movie maker was "a momentary indiscretion," as she battles to save their three-year relationship.
But after new photos emerged showing both of them enjoying cosy dinner dates in Berlin and Los Angeles over several weeks, Pattinson "hit the roof," and now believes that the pair may have been seeing each other since at least May.
"I can't believe a word she says," Pattinson said to a friend.
Kristen Stewart's hooked up with 'Snow White and the Huntsman' director Rupert Sanders - who is married and has two children.
"Kristen is absolutely devastated. It was a mistake and a complete lapse in judgment. She wasn't having an affair with Rupert. It was just a fleeting moment that shouldn't have happened. She never meant to hurt anyone. She's a good person who just made a bad choice," People magazine quoted a source as saying.
Us magazine reported the 41-year-old and his dark-haired woman were caught in a series of clinches on July 17 and "it seemed like they couldn't get enough."
The magazine later published photographs of a "marathon makeout session" between Stewart and Sanders.
Stewart, 22, confessed via People magazine that she had cheated on longtime boyfriend and Twilight co-star Robert Pattinson, 26.
"I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected," said the actress.
"This momentary indiscretion has jeopardised the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry," she added.
Rupert Sanders also issued his own statement, saying he was "utterly distraught about the pain I have caused" to his wife of seven years, model Liberty Ross, 33, and their two young children.
"My beautiful wife and heavenly children are all I have in this world. I love them with all my heart. I am praying that we can get through this together," said Sanders.