Can you tell if your horse is "balanced" when you ride him straight? In other words, does one side feel higher than the other under your seat bones? Does he lean on one rein more than the other when going straight? When he turns, does he lean his shoulder in (while turning his head out and leaning on the inside rein)? If so, he is not balanced evenly.
It will take a little while and patience to correct this and even to learn the feel yourself. When you feel him balancing more to the left (for example), try dropping your right seatbone a bit (don't exaggerate). He will want to balance under your weight so he will naturally begin shifting his weight over to his right side in order to balance under you. Don't lean, just drop your seat bone until you feel him become balanced evenly, then sit evenly yourself. Get used to feeling how he is balanced when going straight and correcting him if he is putting more weight on one side or the other.
On the curve, it sounds like he wants to balance to the inside. A good thing to do is to practice with lots of small round figure 8's at a slow relaxed trot. Watch your weight/seat bones. Observe how he is balancing and try to always encourage him (with your seat bones) to balance evenly (upright, not leaning). The change in direction in the middle of the eight will encourage him to stay upright and not leans. Do a round circle one way, then be sure he takes a step or two very upright and balanced in the middle of the eight, they turn in the other direction to do the second circle of the eight.
Don't try to do too much with the reins, except to "hint" as to the "flex" (bend) that you want. Ride straight at the trot, then hint with your inside rein (small tug and release). Feel for your horse to shift his weight slightly to the outside and curve his neck/body slightly to the inside. Your outside should only keep him from turning his head too much, so he knows you want him to keep going straight and just "flex" to the inside, not turn. Once you've practice on the straight, do the same on the curve. Hint with the inside rein to get him to flex his head slightly to the inside. Be sure not to HOLD his head to the inside. Just a little tug and release, repeat only as needed to "remind" him to stay flexed to the inside. The outside rein should be preventing him from over bending, so it should have constants steady light pressure. Work toward being able to ride the circle with virtually no inside rein (just and occasional reminder).
I hope that makes sense. It sounds like your horse really doesn't understand how to balance when turning and it's best to take a few steps backward and teach him this first, rather than trying to make a major correction which he's doing it all wrong (counter bending and leaning on the inside rein).
Last edited by Myya; 05-05-2013 at 12:27 AM.