To be so upset and embarrassed

Internal interviews are hideous. Be kind to yourself while you regroup and then onwards it is.

No one rational will judge someone's ability in their profession on a bad interview if they know them to be good at their job on a daily basis Op!

I feel for you op. Were you the only internal candidate? If you were I would think it wouldn't have hurt the process to allow you to continue to the end stages. It would have been more tactful ...you wouldn't have felt you had lost face with colleagues you have to work with ....and it wouldn't have changed the outcome management wanted. They could have fed back to you honestly but privately that on the day you weren't saying a close second. You could then have taken stock..Accepted you were to ill on the day to show your best work and decide your next moves ...whether to stay in the role you have for a while or apply for promotions elsewhere. Go back in when you feel better with you head held high and bear in mind that it can be harder to get promotion where you are than applying elsewhere. Good luck with your future planning. 💐

We all have bad days, it’s horrible that it was an internal interview but, as someone who does recruitment, it’s sometimes kinder to stop the process early if you see someone is really struggling. Let yourself feel embarrassed (though there’s no reason to be), shake yourself off and think about what your next move might be. I’d also have a chat with the chair of the panel, explain you appreciated the opportunity but were just too unwell on the day to perform at your best. They may be able to offer you feedback or support for future applications.

Yep. You were ill. So not on your usual form. Write to them and explain that you'll be applying again if there's another opportunity.

Similar happened to me OP, awful at the time but all you can do is carry on, you can't change it now so take the lessons learned but don't dwell. I'm of the opinion that employers should grow the talent they have, it's a shame this doesn't happen more.

Fwiw I'm now in a much better job but had to move organisations to get it.

Cut yourself some slack, Op. if what you've caught is anything like what has been going around my office then it's really nasty. I've been wiped out with it all week and definitely wasn't on top form the couple of days before the symptoms really started.

Add to that, as you say, you jammed too much into the lesson (so you know what not to do next time), and it was a class you don't usually teach - hard normally, and I'm guessing you won't have been firing on all cylinders. Hopefully the feedback you get will help you understand what to focus on next time so you don't make the same mistakes.

Interviews are tough. The formal nature and the stakes immediately pile pressure on. Some people just interview better than others, but it doesn't reflect your ability as a teacher. Prep and practice, learn from this one, and focus on today, not yesterday.

Feel better soon!

You were coming down with something. It's unfortunate but it's not your fault. It happens. It says nothing about your ability as a teacher. Get well soon.

Take the positives out of it, you already know where you went wrong. It’s not a bad thing to get things wrong as you know yourself, you can learn from them. Did you feel valued before this interview took place? You can’t blame them for not performing your best on the day sadly. Just go easy on yourself and have today to go over it then pick yourself up and dust yourself off.
if the feeling of not being appreciated has lingered for a while then the promotion is irrelevant as perhaps it is time to move on anyway?

I've been there. Not in a teaching scenario, but in corporate life. It's (unfortunately) all about what happens in the interview.

The reality is, you just have to dust yourself off, and have another go when the opportunity arises. At least you know roughly why you didn't progress, it would be much worse if you'd given your best and still didn't progress.

I have no words of wisdom to make it any easier, other than to say that iron does not become steel until it has been through the fire. Just look at it as being your path to becoming steel. You'll get there, just don't stop trying 👍