Justin Bieber -- I've Been Covering Up Who I Really Am ... Begs for Mercy

Justin Bieber is making a very emotional and thought-provoking apology to his fans -- but what's unclear is exactly WHY he's doing it.

Bieber seemed to be pouring his heart out in the video he posted late Wednesday evening. He copped to covering up his true feelings for the past year and a half ... and promised he was trying to change.

The video mea culpa came hours after Justin taped an episode of "Ellen." Sources at the taping tell us Justin kinda bombed the appearance -- giving Ellen DeGeneres one word answers ... and seeming extremely nervous.

In his confession, Justin says "I was afraid of what people are thinking about me right now."

JB added ... he wanted everyone to know he's not the type of person to say, "I don't give a f**k" -- or at least he isn't anymore. Watch the video ... makes you wonder how badly he really tanked.