This is how I see the difference:
You’re good at stuff, are confident in what you do, but you don’t let that change the way you treat others – that is, you have an ego that can fit through a reasonably sized door.
You’re good at stuff, but believe erroneously, based on internal analysis of the available evidence (and tainted by your own perception), that you’re terrible at it compared to your peers, and nothing will convince you otherwise.
This craters your confidence and self-esteem, and will also make it so that you tend to buckle in the face of conflict or resistance. It saps your courage, and reduces your grit.