How to be in that zone of interviewing?

I have got a problem. My work life balance is pretty cool. When I am not working, I invest time in relaxation - doing yoga, reading fiction books, learning new languages, cooking etc.
Now when I interview I found that some things that are known to me, I stumbled to answer (mostly related to design) that’s because I am not in the zone quickly for a mere 12 hours of 3-4 days of preparation. Even if I prepare each day I am sure this won’t improve.
Is that happening because my work and life are balanced? Ist not only cracking interview or only technical skills?
After each interview I feel oh shit, I knew this. Probably interviewer was looking for this solution.
My friends say I know many stuffs and I should get better level. Idk what level is that if I give importance to making a balance.

What would you suggest? Do you see a problem here?

YOE 10
TC 76K Euro