Selena Gomez and Justin Bieber arrive at the 2011 American Music Awards at Nokia Theatre L.A. Live on November 20, 2011 in Los Angeles, California. Credit: Steve Granitz/

No wonder Justin Bieber has been acting out: Days before the pop star, 18, was caught smoking pot in shocking TMZ photos, he and girlfriend Selena Gomez have parted ways yet again -- this time for good, multiple sources tell the new Us Weekly, out now.

Although Bieber and the singer-actress, 20, journeyed to Puerta Vallarta, Mexico for a New Year's trip, their happy holiday was cut short when Gomez abruptly flew home to Los Angeles on Dec. 30. "They had another huge fight, and Selena won't forgive him," an insider tells Us Weekly of the perpetually on-again, off-again pair.

It certainly seems final this time -- with Gomez spending New Year's Eve at an L.A. house party with pals, while Bieber rang in 2013 with rapper pal Lil Twist at L.A. club Couture, where he flirted with various girls. "He's been really down about the breakup," a source admits.

Citing Bieber's "bad influence," another pal insists: "Selena is done with him." For more details on the breakup, pick up the new issue of Us Weekly, on stands Friday.