AceShowbiz -

Justin Bieber is partly to blame for Selena Gomez's downward spiral. According to a source, Selena's relationship with Justin led to problems with drinking and partying after they were first linked in 2010.

"While Selena does suffer from lupus and other health issues, that's not the reason she went to rehab," the source reveals, "Her biggest addiction is Justin. She became a different person around him. She was extremely jealous and possessive and felt like she had to keep up with him to keep him. So she would drink and smoke, and then she got into weed and partying."

Selena canceled the remaining dates of her "Revival" tour and went on hiatus on social media after fighting with Justin over Sophia Richie on Instagram. The "Good for You" songstress vowed not to contact the "Cold Water" crooner while receiving treatment at rehab, but she eventually reached out to him.

"Justin is such poison to Selena," another source tells Life & Style, "Even when he isn't around, she is still self-destructing because of him. Her addiction to Justin is what everyone is afraid is going to take the ultimate toll. Justin takes her to a dark place mentally, and she really can't let go."

A rep for Selena hasn't responded to a request for comment.