Confidence is often treated as a personality trait, but research on self-efficacy suggests it is built from specific sources, and some of them can be trained.
The main sources described in self-efficacy theory:
Mastery experiences: having done it, or something close to it, before; usually the strongest source
Seeing similar others succeed, such as a teammate of similar level
Encouragement and believable feedback from people you trust
How you interpret your physical and emotional state, for example reading a fast heart rate as readiness rather than fear
A practical tool: keep a short “evidence log” during training. Note sessions that went well, hard things you managed, and progress markers. Before a competition, read through it.
It turns vague belief into concrete proof, which tends to hold up better under pressure.