The stars have been on-and-off for the last few months, initially splitting after it was claimed Justin had been flirting with models backstage at the annual Victoria's Secret Fashion Show.
Last week, they were further rocked when a report alleged he had been hooking up with Rihanna. Although both Justin and the Barbados-born singer denied the claims, Selena apparently realised she could never fully trust Justin again.
The Canadian heartthrob has hit the headlines a lot recently, when he was pictured smoking what appeared to be marijuana and driving his car too fast. Selena was apparently fearful he was going off the rails so wants to help.
"She's been really worried about Justin and his out of control actions - even his manager, Scooter [Braun] asked Selena to step in and talk to him - so she's been trying to be supportive in the past few weeks and they've been seen together a fair bit," a source told Look magazine.
"I think Justin was hoping there would be room for a reconciliation, but there claims seem to have blown any chance of that."
Although both Rihanna and Justin have laughed off the claims, they apparently got Selena thinking.
"This situation must have made Selena feel she can't trust him," the source added. "The rest of the world might think it's ludicrous that Rihanna would've hooked up with Justin, but if Selena can't be sure, how can she get back with him?"
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