Well, you've all been wonderful candidates but in the end we picked..." You get home with a headache and start the traditional esteem-lowering self-flagellation. It isn't worth it. First of all, everyone gets nervous.
If you avoided obvious signs like audibly knocking knees and throwing coffee at the interview panel, it wasn't that. The odd 'erm' between sentences is fine.
There may be a few people who consistently interview well, but that's usually it: they interview well - and frequently - as they power upwards leaving a trail of partial achievement. Remember, interviewers can have bad days too: the school governor who asks a stupid question then looks across to the head to see if your muttered response is right. There's almost always one who could be an axe-murderer too. Probably is, in fact - you wouldn't have wanted that job anyway.
More to the point, the job was probably not yours - the application process turned up the candidate the governors think is best 'on paper'. If she performs as expected she gets it - you were there as back up. Remember:
* They expect you to be nervous.
* If they don't encourage you, then it isn't a nice workplace.
* You may well have been fine at interview; they just wanted someone else.
* Sooner or later the job will have your name on it.