a couple of questions... are you talking about when you're going over the fence? Or when you're turning? Or just in general? Are you just hanging on it or you use too much?
I had the absolute WORST habit of pulling too much on my inside rein for years! It was a horrible habit for me to break. First of all, do you know WHY you do it? For me I had some incorrect teaching growing up and my trainer had us all using an indirect rein to bend our horses. (no leg, just indirect rein). Unfortunately this does not properly or effectively bend your horse, just cranks their nose to the inside! There weren't any specific exercises that helped me get over it, it was a combination of learning to really use (and understand) my aids correctly and constantly remind myself to get off my inside rein. And (like so many things) it starts off by getting it down on the flat. Once it is fully established on the flat, it will get better over fences. Remember that you ride every horse from your inside leg to outside rein. Part of my problem was that I wasn't being effective enough with my leg and trying to ride too much with my hands. I tried to think of it as wrapping my horse around my inside leg and 'filling' up the outside rein. And at first someone told me it would almost feel like I was hanging on the outside rein (note, she could tell me that because I have very soft hands and always maintain contact on their mouth, I wouldn't say that to someone who wasn't able to maintain a soft contact). There are days when I purposely put a loop in my inside rein to force myself to use my other aids. Another idea that helped me was the idea of feeling both sides of my horse's mouth as I rode and turning was kind of like turning the handles of a bicycle. You must be at the point where you are able to keep a constant feel on your horse's mouth the entire time for this concept to really work. Another idea, make sure that as you turn you're properly using your outside leg and outside rein as you turn, your inside rein simply opens to allow them to go there. Make sure that as you turn with your inside hand you're never pulling back, only out to the side. Sorry this is long. I have a billion things to say on this subject as I am a pro at using too much inside rein. :) And since I haven't actually seen you ride I don't know to what extent (or why) you're doing it. Fortunately I have gotten a lot better, but there are times on certain horses or certain situations where I am still tempted to use too much rein!