Stress is a pop quiz on whatever you actually practiced, not what you planned to practice.

Under pressure you do not rise to your intentions. You fall back to whatever has already become automatic, because the plan you made and the calm you meant to have both require thinking, and thinking is the first thing pressure takes. What is left is the version you rehearsed often enough that it runs without you. That is why preparation is so boring and so decisive.

Look at where you expect pressure soon and practise the exact thing you will need, badly and repeatedly, while nothing is at stake. Rehearse the first thirty seconds most of all.