I have taken so many longe lessons to help my position, strengthen my core, and sit the trot....
I am so thankful for the skills I learned and I highly recommend taking some lessons from your trainer on the longe line. Being educated on the horses physique and how the correct muscles should be developed, I realize that if you have a strong core, as well as an independent seat, we need to look at your horses " imperfections. " If the horse has the proper " lift " of their back, this confirms that they are coming from behind and using the proper back muscles to carry the rider. The well balanced rider will find the feeling compared to sitting on a bouncy ball. This is what is meant by coming" through" or "over" the back and you are not distracted by dramatic motions from the rider, horse and rider seem as one.
If the horse is using the appropriate back muscles because he was correctly trained but the rider is lacking in the skills to stay with the horse,, the horse will soon be contracting the back muscles in hopes of compensating for the riders lack of balance. Perfect practice makes perfect but only if the practice is beneficial to the horse.
" No one can teach riding so well as a horse " - C.S. Lewis