Your confidence in your ability to make decisions should reflect how good you are at making decisions! – the ‘presentation’ should reflect the ‘content’. How good you have been historically at making decisions should inform the ‘courage of your convictions’. As you make more decisions and test yourself in the real world you learn how good your decision-making is.
But people often over or underestimate their decision-making skills – some people’s confidence is greater than their ability, and with others their decision-making is better than they give themselves credit for (n.b. the introverted).
Those who overestimate their abilities need to be reined in (it is unlikely they will rein themselves in); those who underestimate their decision-making need to get over themselves.