To get your medium trot off the forehand and coming through more from behind try the following exercise:
On the long side of the arena start in shoulder-in. About a third of the way along straighten up and ride 4-5 strides of medium trot then return to shoulder in. The shoulder-in provide collection, lightens the forehand and engages the hindquarters and makes sure that the inside hind leg is stepping right under. Make sure you are back in shoulder-in again before the medium trot loses its quality. 4-5 strides is plenty to start off with, you can gradually increase this as the medium trot improves.