“Twilight” heartthrob Robert Pattinson is “heartbroken and humiliated” after his girlfriend and co-star Kristen Stewart was caught cheating on him with her married director.

Sources tell us Robert was stunned when Kristen, his paramour of four years, admitted she’d been caught kissing her “Snow White and the Huntsman” director, Rupert Sanders, who is married with two young children.

Witnesses saw Rob and Kristen fighting backstage at the Teen Choice Awards in Los Angeles on Sunday, just days after the 22-year-old actress and Sanders, 41, were pictured sharing a steamy rendezvous.

A source told us, “Rob and Kristen were having a long and intense conversation backstage, and he looked miserable, heartbroken and humiliated. She was pleading.” Another source said after learning she’d been caught in the act, Kristen had begged the photographer not to publish the pictures, then was forced to own up to Rob late last week, adding, “Rob is deeply in love with Kristen and is very jealous when other men hit on her. This will crush him.”

Us Weekly reported that Kristen left the Hollywood home she shares with Rob on July 17 and spent the afternoon driving around LA with Sanders — who’s wed to British model Liberty Ross — “in search of secluded places to make out.” They were pictured kissing in a car and later canoodling and hugging at a park in what was described as a “marathon make-out session.”

Yesterday Kristen issued an astonishing apology. She said, “I’m deeply sorry for the hurt and embarrassment I’ve caused to those close to me and everyone this has affected. This momentary indiscretion has jeopardized 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.”

Sanders also put out a statement: “I am utterly distraught about the pain I have caused my family. My beautiful wife and heavenly children are all I have in this world. I love them with all my heart. I am praying we can get through this together.”