Quiet preparation turns a big moment into a normal one. Loud people call it luck.

Nerves come mostly from unfamiliarity. If you have already done the thing ten times in private, the real version is the eleventh, and your body treats it as ordinary because as far as it knows it is. From outside this looks like natural composure, since nobody saw the ten rehearsals. They only saw the calm.

Rehearse the part you are dreading until it is boring. Say the speech out loud, run the conversation in your head, do the problem type until your hand knows it. On the day, you will not be performing, you will be repeating something you already know how to do.