Selena Gomez is still torn up about her ex, Justin Bieber -- or she's a really good actress ... that's the only explanation for her sappy and tear-filled performance last night in Brooklyn. Selena started sobbing and weeping near the end of her ballad, "Love Will Remember" -- which is widely believed to be about the Biebs ... partially because the original leaked version of the song featured a voicemail Justin left for Selena.