A former colleague from my previous school once told me he couldn't understand how I managed to look so calm and unflustered when talking to a largish audience of parents about the school's programme for its first year students. I'd sort of been selected as the spokesperson for the school in my third and final year there since it was generally agreed that I looked calm in pretty much every public situation, almost as if I was enjoying myself and this would go down well with what nowadays gets termed 'stakeholders'. My answer seemed to take the questioner by surprise, and took me a moment or two to figure out myself since I'd not really ever asked that question of myself at the time in question.
I'm just pretending to be calm. I'm a pretty good actor and it seems to work. (Not sure that that's exactly what I said, but it's close.)
What I didn't get into is the weirdness of acting and the way you (well, me, anyway) start to feel exactly what you are pretending to feel. To this day my pulse races - I think - before speaking in public (except in front of a class) so the nervousness, the being flustered, is real. But it disappears once the act begins.
I sometimes wonder if physical fear might disappear if one pretends to be brave. But I'm too much of a coward to try that one out.