I'm terrified of flying and really don't see how a course can help. The reason we are scared is because of crashing, I just don't see how you come to terms with that.
I know you're more likely to get killed on an escalator or in some other obscure way but the fear remains.
The only thing I can offer you is that you can't be worried for 12 hours. I've been to Thailand 3 times and on each occasion the fear has waned within the first couple of hours, you won't have the energy to keep it up for that long.
Good luck and enjoy Thailand.