It's amazingly hard, before you're published, to think beyond that glorious moment. But one thing that many aspiring writers know about and are horrified by is how "these days" (Dickens of course being "these days") authors must do all sorts of events, appearances and readings. Since writing has an unsurprising habit of attracting people who are very happy to spend large amounts of time on their own, and who find themselves more eloquent on the page than in person, many of them are terrified.
Performance nerves are entirely natural: it's a mild case of fight-or-flight. Actors, musicians and dancers feel it, and so do teachers, barristers, best men and captains briefing the platoon. It sharpens your reactions, narrows your focus and makes you want to pee. It uses up blood sugar (which was why doughnuts were so welcome in Mexico) to give you extra energy, and the day you don't feel keyed-up and a little nervous before a gig is the day you'll stop performing well. If you can trust that the slight flutter in your stomach won't lead you to say anything daft, or dry up (it won't), then you can safely settle in for the show.
I feel keyed-up and alert before events, but I don't (so far) get nervous. But I do know what the true, paralysing, brain-fudgifying, hand-shaking, sick-making, tongue-tying stagefright is like, because it was a large part of why I wasn't as good an actor even as I might have been. On the other hand, I have several writing friends who used to be actors and felt no more than respectable stagefright when playing a part, but for whom reading their own work and talking about it is pure horror. And I was just brooding on this when an aspiring writer confessed that she was terribly nervous before taking part in the Getting Published day, even though she's someone who "doesn't get nervous". The thought of discussing her work with a book doctor (who wasn't me, but could have been) was terrifying.
That disabling terror is different. A clarinettist friend sought professional help when it threatened to destroy his career. The physiological stuff that Wikipedia lists was unmanageable, and yes, a fear of wrong notes and missed entries is sensible. But of course he'd rehearsed enough, so why does sensible anxiety become overwhelming terror? The terror of What the Book Doctor will say is a clue: to stand up and do your stuff lays you wide open to the judgements of others. That judgement may be that you're Bad, says your unconscious, anxious from infancy to please parents and pseudo-parents by Being Good, because what will happen if you don't please them is too terrifying; to a psychotherapist it's not ridiculous to talk of fearing judgement as fearing anihilation. Writers are less likely to have sabre-toothed music teachers or carnivorous critics in the audience, just twenty (or 200) well-behaved, well-disposed people, so the fear seems quite disproportionate to our situation. But infant fears know no proportion: this is a small piece of an ancient, fundamental terror.
There are, however, practical things you can do about it, starting with with checking out the venue and asking as many questions as you like, continuing with a few breathing and voice exercises, and ending with remembering to pour some water into your glass while you're being introduced, so you're not faffing with the bottle top later. There are more psychological things you can do too: tackle your Inner Judge. But that's a whole new post.