The 34-year-old partied hard at New York's Marquee club with her new mystery man last week. The actress seemed smitten with the man, who appears to be closer to her own age than 25-year-old Justin.
Across town, JT was having just as much fun without Cam while promoting his new album FutureSex/LoveSounds.
"He had a listening party at G Spa ... and his eyes were wandering," says a pal of the singer. "He has not talked about her [Cameron] at all and comes off like a single guy."
Cam feels she has given the three-year relationship her best efforts, even consoling Justin after Edison, Alpha Dog and Southland Tales – all flopped.
"Justin's worked so hard," says a source. "He's proud of the effort he put in and all three performances, but he can't help be disappointed at how things have gone.
"Cameron has been so supportive, though. If it wasn't for her, I'm not sure he would carry on with the acting. It's reminded him of just what an amazing woman she is."