Basically it sounds like a possible submission problem. You cannot move your horse's body around easily because he is ignoring your aids. I hate to ever blame the horse, because it's usually a rider error. As with every other problem, always eliminate a physical problem. After that is done, you need to take it back to basics...at the walk work on leg-yields, sidepasses, flexion and counter flexion so that he listens to your leg. If he is dead to your leg, I'd recommend a mild spur.
The big issue with not having good circles and straight lines is because your horse isn't balanced between the reins and your legs. So you yourself need to be balanced in the saddle. That way you can help him out. Like said above...get him so that you can easily move any part of his body around so that you can control the circles and lines.
Not sure of you know any dressage but shoulder-ins, half-pass, hauches in and haunches out are all great lateral exercises which incorporate getting him off your leg.
DRESSAGE, n.: the passionate pursuit of perfection by the obsessively imperfect -Author Unknown