JUSTIN TO SELENA ''I WISH I KNEW HOW TO QUIT YOU''

Justin Bieber is dating lots of chicks ... and he says he's not back with Selena -- but it sure seems he wants the option.

The Biebs posted this pic of him with S.G. back in the day ... it looks like 2013

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As for why he did it ... reports surfaced Selena was macking One Direction's Niall Horan at Jenna Dewan-Tatum's 35th BD party Friday night at Bootsy Bellows.

It's like Jake in "Brokeback," ... right

#justinbieber #selenagomez