Increased pressure and ticking clocks, failures and mistakes, relationship friction, duty and responsibilities, no-win situations

Self-doubt is difficult to deal with at the best of times. But when it strikes during high-stress events or moments when decisiveness and quick thinking are imperative, the stakes go up dramatically. To complicate matters, consider adding a healthy dose of insecurity to the following scenarios.

When a new challenge is introduced after a long battle
As the character is getting ready to propose
When the character is about to take a leap of faith to try something new (singing tryouts, apply for a new job, etc.)
When the character needs to take charge 
Right as the character needs to do something dangerous (leap out of a plane, jump off a bridge, or run into a burning building)
While participating in a murder, heist, kidnapping, etc.
When delivering an important speech
While trying to lie their way out of a situation
When an obstacle presents itself
When others are relying on the character to follow through
When the character needs to take on a big responsibility
When it's important to speak up
When there's an opportunity to impress someone
When someone needs to be protected

Delaying, and missing an opportunity
A competitor getting the edge
Missing the perfect moment
Losing a window of much-needed privacy
Creating the seed of doubt in others (about the character's abilities, leadership, commitment, etc.)
Having to reschedule
Never getting a chance to say something to someone important
Disappointing a loved one