Don't worry - going into stiff defensive mode is absolutely a normal reaction for any person! It usually takes a lot of years and a lot of experience riders difficult horses to develop control over this automatic reaction. I have found that once you can control this instinct and can ride out such behaviour as a spook, buck etc. it is quite easy to get the horse settled again. Many riders will try to overload the horse with aids, legs, rein,, seat all at once while trying to hang onto and correct the behaviour at the same time. If the behaviour was a genuine flight relex, the best thing you can do, is to try and ignore the behaviour. Just sit quietly, relax your knees and thighs, and gently keep your calves on the horse's sides. Give your reins a little, and just let the horse work it out, don't interfere with his balance, just sit and go with it. You are the leader, and if you show that you are scared as well by gripping, pulling or holding your breath, the horse has every right to continue trying to get away from whatever spooked it. Show the horse that you're not worried by continuing with what you were doing, and I promise you, the horse will settle quite quickly :)
In saying this - it DOES take years in the saddle and experience with these behaviours to hone your skills to that level. I also think, that this type of horse is not so bad as a riding school horse. I am under the impression that all riders need to learn how to ride out some 'natural' horse behaviour rather than sitting on an old plodder constantly. Little spooks, humps etc. teach you how to sit quietly and ride a horse forward.