Justin Bieber -- Acting Out Because of 'Heartbreak' Over Selena Gomez

Justin Bieber has been spinning out of control not because he's trying to "act like an adult," not because his friends are a bad influence ... but because he's suffering his first case of heartbreak -- courtesy of Selena Gomez -- so say people who are in regular contact with the singer.

Our sources tell us ... Justin was "torn apart" when Selena pulled the plug on their relationship in December, when she walked out on him in the middle of their Mexico vacation.

We're told Justin feels "tortured," because Selena has not made a clean break -- far from it. We're told she is regularly calling him and sending him mixed messages, and it's been messing with his head.

That could explain why Justin went to Selena's L.A. home last week, but hasn't been seen there since.

Our sources say ... the pot smoking in Newport Beach, the reckless driving and the fighting has all occurred after the split, and they say it's very clear that Bieber is holding a lot of anger that he's releasing ... often inappropriately.