Stress management in a chess tournament was addressed in this forum by Ritesh Singh Feb 1, 2018. The following is nearly verbatim from his comment.
It can be captured in one word: Equanimity. Or, in other words, detachment with results. You should try to make yourself as unconcerned
as possible with the result of your game / match / tournament.
This is ...
I would say figuring out under which conditions the anxiety kicks in can be an interesting challenge for you, which should help you figure a plan to deal with it.
clock vs. no clock - really no difference?
vs. friend and vs. stranger - any difference?
OTB vs. online
you say puzzles/tactics are not really a problem; how about competitive puzzle-solving ...