Both Arabians & TB are very aware of their riders body language. Horses are also herd animals who have a long history of taking their cues from the head horse, with “head horse” status passing to the rider for horses being ridden. This means that on a more highly strung horse, it is critical that the rider be able to control the amount of tension is his/her body. Any increase in tension tension will tend to be interpreted by the horse that the “head horse/rider” has seen or perceived some threat which is the more concerning to the horse being ridden because he can not determine what the “head horse/rider” is concerned about, and therefore the horse does not know from which direction the threat is coming. It never occurs to the horse that it is his/her actions which are making the rider tense. This will not be easy to learn, and it might be easier to try it out on a friends less sensitive horse until you get the hang of it. The horse was probably more relaxes when riding in the field because you were able to relax. Particularly check for tension through your heavy back muscles, and in the muscles from your fingers to your shoulders.