If you need to think about what to do you will be lost. An example - try to hit a tennis ball while thinking about it consciously. Think of the unknowns, speed, curvature, trajectory, spin, bounce, deceleration rate, angle of deflection, racquet speed, string tension etc - all unknowns. Practice, practice, practice. Much as soldiers need to learn to perform under stress, so must we.
Business is a war after all.
Another factor is that under stress we are most likely not to 'be all we can be', as the US army puts it, so we must react automatically based on a practiced ritual of behaviour. Practice can also be 'mental', as our subconscious can't differentiate between imagined and real experience. Reactions are subconscious so we can and should start by imagining ourselves doing well in unfamiliar situations. Honesty will also help reduce fear. In the 2000 US presidential elections both candidates have talked openly about embarrassing issues, personal shortcomings and past indiscretions.