Selena Gomez -- Cries Onstage During Song (Allegedly) About Justin Bieber

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.

She also goes into an emotional thank you to her fans, but since turning on the waterworks seems to be her thing -- we gotta ask ...

Here's something Selena should really be crying about -- this awesome moment from "The View" this morning ... when the ladies intro'd her to start singing, and ... well, watch what happens.