This is key. Worry when it has happened, and it's long over. Don't worry about a million hypothetical situations beforehand. Analyze the one situation that does happen, after it has happened.
This too. It took close to 5 years of competitive sport karate before I was able to zone out before/during a fight. Now it comes naturally and I can do it at will for anything; homework/studying/training/doing dishes. Of course, when asked "how do you do it?" I always respond with "iunno, it just kinda happens". At least for me, I can't explain the sequence of events that takes place in my head to zone out; one second I can see and hear everything, the next I'm in a silent, pitch black world with just myself and my opposition.
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