Anyone moving from Western Riding to English is likely to have trouble with sitting trot unless they are lucky enough to live close by to a riding school where there are to be found some school master horses. It is as much if not more the horse that teaches the rider than the instructor.
The speed of the horse in trot for such an exercise early on should be slow - no more than a jog. Secondly the rider should seek help from a friend to lunge the horse from the centre of the arena, thereby taking away the responisiblity for controlling the horse whilst trying to learn to sit in.
The dressage rider you see sitting in to a sprauncy horse with head gently nodding is a very experieinced rider.
The Spanish Riding School suggests it takes a year for a new applicant to the school to learn to sit to the trot properly. Don't be impatient , in the meantime get the rising trot firmly established and the sitting trot will the follow with time and experience.
Practice, practice, practice - perhaps just a few lengths at a time. B G