I'm late to the party as usual!
I think you have a very good grasp on the basics and someone has obviously preached position to you. Your leg is laying nicely in a dressage position and you're sitting upright with your shoulders back. The first thing a rider should learn is how to follow the horse and stay out of its way. I think you have that part down, so now you need to learn how to influence the horse in a positive way.
This stage is what I call the "being an effective rider" stage. We push the horse to do better work, developing the horse using the training scale. You are doing the right thing by having a trainer help your horse in this department. That way you can supplement his/her riding so your horse can continue to move forward in its training.
To help your horse the most, you need to stop trying to look perfect because you are trying to hold so still you are rigid.
Hands. Your hands are fixed and are not giving. Think of what it would be like if you were fishing and you hooked a big, heavy log instead of a fish. You need to start a conversation with her. Shorten your reins just an inch so you have a more consistent feel on her mouth. Focus on one rein at a time. This is what we call uni-lateral aids. Squeeze your fist on the rein and feel the corner of her mouth and let go. Like a leg aid, we pulse and never hold because that makes our horse's ignore us. Then do the other side.
Strengthen your shoulder girdle and really pinch your shoulder blades together so you can relax the rest of your arm. Shrug your shoulders with the rhythm of the trot to shake out some of the negative tension.
Move your hands and forearms with her head. Erik Herbermann says to keep your elbows "pointy", which means pointed down, but the horse owns the hand and forearm. You need to allow your elbow joint to open and close so the horse can feel that lively connection through your hand. The same goes for posting with your hands. The hands stay with the horse's head and the elbow joint opens and closes. Your hands should not move up and down with you. When you hold your hands still and fixed, the horse is easily able to pull you out of the saddle. So you are not anchored quite right into the saddle.
Seat and leg. Ride with your seat bones pointing forward, not back. They are sort of out behind you right now, which leaves you with a weak position and also blocks the horse's hind end from really coming through.
You have too much weight in your stirrups and not enough in your seat. Part of the reason, especially at the posting trot, is because you don't have enough energy and feel like you have to propel yourself out of the saddle just to get up. This is backwards. You need to propel the horse forward. Then the horse's energy will push you up out of the saddle and you simply allow it and then come back down receiving the energy. Think "squishy buns" to help relax your cheeks. The tension in your shoulders goes down through your back and to your seat.
Be careful not to push the horse too forward causing it to rush and come out of balance, but rather just to the edge. Try to feel the energy of the haunches moving under your seat over the back, moving forward into your hands.
Do not collapse forward through the middle in transitions. Think of yourself as a banana. Your head is the stem and the rest of you is the curve. You are curving with your seat furthest from the horse. Flip it around! Push your seat towards the front of the saddle (instead of sliding out the back) and get your shoulders and leg behind it. Over-exaggerating this image in your head and trying to feel it in your body should get you about straight where you need to be. Keep this image in your mind while you canter so you don't slosh to the back and then lean forward to try to keep up.
Don't worry about "framing her up" too much right now. Just keep that elbow soft and hands giving having that conversation with her. Ride her forward with energy riding her with a more influencing but relaxed position.