The couple met on the set of popular movie series "Twilight" and have been dating since 2008.
The heartthrob was reportedly shocked to find out that his girlfriend had been caught kissing her 'Snow White and the Huntsman' director Rupert Sanders.
Sources close to him had revealed that "he is struggling to cope with the news". One of them witnessed the couple arguing backstage during the Teen Choice Awards in Los Angeles on Sunday.
The source revealed that ''Rob and Kristen were having a long and intense conversation backstage, and he looked miserable, heartbroken and humiliated. She was pleading.''
Another source also admitted 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.''
The 22-year-old actress has since issued a public apology to say how sorry she was, after pictures of the scandalous couple had surfaced.
The 41-year-old directory has two children with his model wife Liberty Ross.
Kristen 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.''