confidence is never solid, never unmalleable, never untested. Even the most naturally confident people will have times in their lives when that confidence is tested, and sometimes shattered or at least, bruised. With out grit, it’s difficult to get oneself out of those situations, and one can spiral downwards from the peak of confidence into the dark shadow world of self-doubt.

Confidence, real or fake, is something one always has to encourage, seek and feel, in order to be successful at anything, but with out grit, the bitter determination in life to constantly battle through everything and achieve one’s goals, confidence is nothing more than a veneer that eventually everyone will see through. A man with confidence and without grit is nothing more than a slick snake oil salesman.

Extracted from this article

But what is Grit?

Grit is passion and perseverance for very long-term goals. Grit is having stamina. Grit is sticking with your future, day in, day out, not just for the week, not just for the month, but for years, and working really hard to make that future a reality. Grit is living life like it's a marathon, not a sprint.

From this TEDtalk

I guess it needs no more explanation. I Hope it helps.