Anger does not respond to events. It responds to the gap between what you expected to happen and what did.
The event is neutral. The expectation is the variable you control — and it is the variable Seneca targets.
Letter 18’s instruction to practice voluntary discomfort is the preemptive intervention. By rehearsing adversity before it arrives, you eliminate the gap between expectation and reality that anger requires to operate. You cannot be outraged by what you have already prepared for.