I looked at the pic you posted and instantly thought of an exercise to feel a different level of balance from your horse. You have a nice connection to the bit. It’s time to put it to good use. Here’s the exercise.
Trot 8 to 10 strides.
Come to halt
Without throwing away outside rein connection, make a 1/4 turn (90 degree) by asking your horse to turn on his forehand (shoulders). You will need to keep that outside rein and leg, but you can give away some inside rein if you need.
Resume trot 8 to 10 strides, come to halt
Lather, rinse, repeat. You’re riding squares. Change direction from time to time to keep your horse guessing. You’ll find a little slack in the reins each time you turn as your horse begins to engage his hind end. Take up the slack but make sure your elbows and shoulders are soft and following. Keep your fingers closed around the reins. The idea is to get you feeling the connection more in your entire torso over just your hands.
Once your horse is bored with the halt/turn, change it to a walking turn, and finally a trotting turn. When you get to trot, you will need to make it a larger more shallow turn until your horse has better balance, but just play with it.
Finally try it on a circle. Think of it as an octagon instead of a circle and make the mini turns on the forehand each point of the octagon. Your horse should respond my reaching more into the bridle as he engages his hind end. Keep thinking about your elbows/shoulders. Make sure there is not tension. If this exercise is successful, you will probably end up with your reins about 6 to 8 inches shorter than when you started.
Have fun with it!