The best way to overcome your anxiety in the classroom is to know your stuff. But even knowing your stuff won't fix everything if you are nervous in front of a crowd.
There is a mindset you need to learn in a Thai classroom and you can pick it up from watching Thai teachers... or some farang teachers who have been teaching for a long time.
Have a wealth of material to draw from and don't rely on what you have been given from your employer. And this applies as much to corporate work as it does to teaching young children.
Generally speaking - all the text books in the world and even those stale hand outs that you have copied from the internet are never enough to inspire/wake up your academic progeny.
These days, Thai ESL students have seen it all before and have had legions of odd looking foreigners traipsing through their lives with their strange ways of speaking and odd choices in foot wear.
My advice to new arrivals who have never been a teacher and having trained for it, is this...
1 - Know your stuff. (Pick up a book on the English language and study it.) You can't teach anything that you don't already know!
2 - Fly to Thailand and go straight to a language center like Inlingua. (There are loads of others to choose from.)
3 - Look at other teachers teaching. (Make notes of what you like about them... and what you don't like.)
4 - After a few months start looking for a permanent job and CHOOSE YOUR EMPLOYER CAREFULLY.