You know, I've always said that people only say that animals are stupid, because they themselves are the ones lacking in brain cells.
It is a very high strung breed, and they are very hot blooded. It also sounds like these are not trained very well.
Misbehaving =/= Stupid
Spooky=/= Stupid
Instead of telling yourself that they're stupid, go with the mind set that they just need more work, and proper training because if you think that they're stupid, and that they're going to freak at everything, and all the other crap that you seem to have decided about them, they will only ever be what you think.
You need to go with the attitude that they're going to improve, and everything is going to go well. Don't be oblivious, you always need to be alert when handling horses, but don't expect it to happen, because then it will.
I also suggest you go over what they're eating, and make sure that it isn't anything hot. Hard to do, because hot foods make it easier to keep weight on, and TBs are notorious hard keepers, but anything hot will just cause problems.
I love Thoroughbreds, and I live in Kentucky where I get to see them every day. I plan on buying a Thoroughbred some day, because they're a fabulous, versatile breed that can do many things if you let them.