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.

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?