"I'm a failure" needs 5 evidence + 5 counter. Write them.

Spontaneous muscle tremor after a position — discharges deep nervous tension.

Repeat the same refusal in the same words calmly, no matter how the other person reshapes the pressure.

Say no in three sentences: thanks, refusal, closing — with no "sorry" or justification.

Memorize one 7-word refusal sentence and rehearse it until it emerges automatically under pressure.

Say "no, I can't" — complete sentence. No explanation, no apology.