The actor is "heartbroken and humiliated" after girlfriend cheated on him.
The heartthrob was stunned when he discovered his partner of four years had been caught kissing director Rupert Sanders - who she worked with on Snow White and the Huntsman - and sources say he is struggling to cope with the news.
One source - who saw Rob and Kristen fighting backstage at the Teen Choice Awards in Los Angeles on Sunday (22.07.12) - said: "Rob and Kristen were having a long and intense conversation backstage, and he looked miserable, heartbroken and humiliated. She was pleading."
Another source added to the New York Post newspaper: "Rob is deeply in love with Kristen and is very jealous when other men hit on her. This will crush him."
After pictures emerged of Kristen and Rupert - who has two children with his wife Liberty Ross - kissing the 22-year-old actress issued a statement saying how sorry she was.
Kristen - who started dating Robert after meeting on the set of the first Twilight film -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 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."
Rupert also spoke out, saying: "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."