I struggle with this too even after competing 8+ years. A few things that have helped me are: 1. Relax it's just 1 routine even if it's nationals falling isn't the end of the world and it's ok 2. Along with number 1 what you think matters. If your brain is constantly say things like "oh, no" "I'm going to fall" or "I can't do this" then your body will react accordingly. Try to thing about how to make the skill better or "I can do this" "it's ok". 3. Practice routines in uncomfortable situations. For me this is with no music or background sounds. For other this could be with a "judge" watching. I still struggle with my confidence on beam but these have helped me lots. The things my gymnasts currently like is "breathe in confidence, breathe out doubt" and "gaslight yourself into being confidence" basically fake it till you make it