The speller who loses on a word they knew learns the most important lesson: nerves need practice too.

Knowing something in your room and producing it with your heart going are not the same ability, and the second one needs its own practice. Losing on a word you had cold is painful and informative, because the gap was never the spelling. Performing under pressure is a trainable skill rather than a flaw in your character.

Practise under conditions closer to the real thing: out loud, standing, with somebody watching. Do it badly a few times on purpose. The aim is to make the nerves familiar instead of new.