I've had this problem for years, ever since I rode a horse who cut in going to the right and I (wrongly) tried to drag him back out to the rail for months on end. Please don't flame me for that; I know it was wrong and I'm paying for it now!
My problem is this: I ride with my left rein shorter and my left hand higher, stiffer, and without as much give. I've tried lengthening my left rein, but as soon as I think about any other part of my body, my hand drifts back to my hip so I end up with the same tension in the rein as normal - just with worse equitation.
To the left, I'm okay. My horse goes like she's supposed to, canters on the correct lead, everything fine. To the right... I can't give up my outside rein enough to bend her to the inside, or even travel straight. It's killing her right-lead.
I've spent way too long beating myself up mentally about this. I need some new exercise to break this habit. Help?