When stressed, annealed glass breaks into dangerous, jagged chunks. But tempered glass can withstand pressure and even impact from flying objects, because it is created under extreme heat. As we know with side and rear car windows, tempered glass cracks but never shatters.
Speakers who strive for word-perfect content are annealing their presentations. Any small interference will shatter the speaker or their talk. Rather than avoiding these stressful situations, bring on the heat. Tough audiences, debates, and pushbacks make you resilient and strong.
In rehearsal, create pressure for yourself. To make sure you know your content without reading or memorizing it, start in the middle or go out of order... Do the entire speech out loud and extremely fast... Best of all, paraphrase your content.
When your presentation is right where you want it, deliver it in front of peers and then accept their feedback... Practice your talk when you're tired or believe you can't think straight... Digitally record your speech... Set a stopwatch for the amount of time you have to deliver your talk. And force yourself to stay within the allotted time... Only then will you be tempered under all circumstances, no matter how challenging.