“I’m good,” said once and calmly, ends more pressure than a long speech.

Pressure feeds on engagement. A long justification hands the other person material to work with, while a short calm answer gives them nothing to argue against and nowhere to take it. Repeating it without heat is the entire technique. Most people let it go by the second time, not because they were convinced but because no conversation is left.

Decide on your short answer before you need it, four or five words at most. Use it the next time somebody pushes, and resist the urge to improve on it.