Composure lets training show up.

You cannot perform beyond what you have practised, and under pressure you often cannot even reach what you have practised, because agitation blocks access. Composure is what lets the preparation actually appear. This is why two people with identical training can perform completely differently when it matters.

Along with practising the skill, practise the state you will need to be in. Rehearse under some mild pressure rather than only in comfortable conditions. It is the combination of the two that shows up on the day.