Self-efficacy is task-specific. Believing you can follow a stop rule under pressure is different from believing you are a good trader, and it is the more useful belief because it points at a behaviour.
It is built mainly through mastery experiences: small, repeated successes at the exact task. This is why starting at minimum size matters. A hundred correctly executed small trades builds a genuine belief that you can execute; one large trade managed well does not.
When it collapses during a drawdown, rebuild it the same way. Reduce size until execution is easy again, accumulate clean repetitions, then scale. See scared-money for what happens when it is missing.
Related: locus-of-control, growth-mindset, scared-money, deliberate-practice