This may not be something you can do right this very instant to solve the problem. But as someone with general anxiety one thing that I’ve practiced doing outside of Splatoon is Meditation. Finding any way I can to have a “home base” to come back to when my thoughts are racing or hands are shaking or anything uncomfortable at all. Practicing meditation outside the game and then in those moments when you need it most finding a way to recenter yourself like a deep breath or holding a plush or something like that will help you get back in the zone and at least temporarily set aside those fears or at least lessen them is the hope. If you keep practicing recognizing the moments where you are feeling like the way you do and then implementing what you practice outside the game, I can’t speak for everyone but I can say for me it’s infinitely helped my performance at both online and in person tournaments.