The brain does not distinguish well between a vividly imagined danger and a real one: the same circuits activate and the body prepares identically. The difference is that a real event ends, whereas anticipation can run for days or weeks.
This explains the common observation that the presentation was easier than the fortnight before it. The physiological cost is not proportional to the length of the event but to the length of the anticipation.
The mechanism that sustains it is avoidance. Postponing a difficult phone call lowers tension immediately, and the drop reinforces the postponement. But the problem remains, and in fact grows, because the consequences of delay are now added to it. The loop is identical to the one in compulsions, only less visible.
What works runs against instinct: shortening the distance in time. Scheduling the feared thing as early as possible narrows the anticipation window, and the action itself turns out almost always to be more bearable than mentally rehearsing it.