That depends what type of problem you are having, ie, shoulders spine, hip alinement or it could be knee or lower leg moving. Perhaps a good instucter on the ground could help.
Do not forget to look far ahead, it will oblige you to straighten, put your shoulders backward and hold your back!
Maybe put off your stirrups and go on extended paces ! It worked for me, I was so destabilized that I had to grip my legs so hard and now my position is much better !
You should put your stick in your back and keep it with your elbows ! You'll have to be straight to still keep your stick ! This way, you'll improve your position and if you do it few times, you'll keep this position as usual :)
First of all, you should put off your stirrup ! To me, the legs are the most important about the position (and the hardest as well!) You have to train yourself again and again, even during 2 or 3 weeks, training only to improve your legs, you have to think about the relaxing your legs pressure. If you do, I think your chest will follow your legs' improving :)