Besides Accomplishments, What Increases Confidence?

Stress leads to anxiety and/or depression, which are deadly to self-confidence.

Not recommended, but the phrase "Hold my beer" is short-hand for someone who's about to do something cocky (if ill-advised).

We pump ourselves up through our favourite tracks.

Dress the part

Similar to #8

Seeing the difference you make in someone else's life gives you a confidence boost.