It sounds like you're having a lot of problems with harness fit. Do you know anyone in your area that is experienced with driving? In another topic you mentioned your breeching being loose enough to move a lot. The whole harness may be too large for your horse.
Any Amish harness maker could sell you a strap with two rings for the reins that buckles on the backstrap. Usually used when the driver doesn't want to have to keep contact with the horse, like plowing. You might be better off to replace your hip strap with one that is shorter so won't need to be buckled so high.
MajorClementine has a great shot of her harness from the driver's seat. Compare what hers looks like to what you see when you're driving.
Just my 2 cents!