You can't gain more confidence

The presentation starts in 15 minutes. You're nervous and looking for more confidence but it's just not there. 

Confidence isn't the only tool for the underprepared. What you need is courage. Courage doesn't feel good like confidence. You take action despite fear.

If you're in this situation, it means you have to learn something. And once you follow through, even if you fail, you'll be more confident next time.