Actor Robert Pattinson attends the "Cosmopolis" premiere at The Museum of Modern Art on August 13, 2012 in New York City. Credit: Taylor Hill/FilmMagic

Put on a happy face?

Though he seemed slightly skittish during a Daily Show taping Monday afternoon (one audience member remarked he was "awkward" throughout his chat with Jon Stewart), Robert Pattinson was determined to see the rest of his Cosmopolis promotional duties through.

Clad in a Gucci suit, the actor, 26, smiled for photographers as he walked the red carpet at the Monday evening premiere of Cosmopolis in NYC. But despite the smiles captured on camera, one witness says, he looked "extremely serious" and "tense" between flashes.

Not that anyone would blame him, considering the heartbreak he's endured over the last few weeks. On July 24, Us Weekly broke the news that girlfriend Kristen Stewart had been unfaithful to him with her Snow White and the Huntsman director, Rupert Sanders.

"Rob's a total mess," one insider told Us after Pattinson went into hiding at pal Reese Witherspoon's vacation home in Ojai, Calif. "He's trying to figure out why Kristen did this."

The actor's having a hard time forgetting about the scandalous photos of Stewart -- whom he began dating in 2009 -- locked in an embrace with another man.

"He's wondered, if he were to ever salvage this relationship, how would he even go about doing that?" the source says. "Rob doesn't know if he should forgive her," another friend admits. "He doesn't know what to do."

Once November rolls around, Pattinson will have to put his personal problems on the backburner once more to promote the final Twilight installment alongside the woman who broke his heart. Luckily, he'll likely have the rest of his costars as allies while the group travels around the world to fulfill their Breaking Dawn: Part Two obligations.

"They'll rally around Rob," says a Twilight source. "He is the nicest guy. They can't believe Kristen did this to him."