Justin Bieber's getting real about his imposter syndrome ... telling fans he's always "felt unworthy" despite dozens of people in his life telling him he's enough over the years.

The singer-songwriter posted the emotional message to Instagram Thursday ... writing that people always told him he deserved everything he received in life -- but, he still felt like a "fraud" despite their kind words.

Justin said he felt "sneaky" ... 'cause if people knew how judgmental or selfish he was, he doesn't think they'd see him the same way.

So, he tells his fans ... if any of them out there feel the same way -- they're not alone 'cause he feels "unequipped and unqualified most days."

Bieber's post comes days after he hopped on social media to tell his fans none of them have anything to prove ... despite the feeling, at times, that they may owe others a lot.

It was a defiant post ... coming at the end of weeks of speculation and online gossip regarding Bieber's health -- including fears he's using hard drugs again.

Justin and Hailey fired back at the claims last month ... with a rep for the couple calling the persistent rumors "exhausting and pitiful and shows that despite the obvious truth, people are committed to keeping negative, salacious, harmful narratives alive."

Unclear why he posted the latest message ... but, it certainly seems like all the drama following him may be taking a toll.