Does your horse know how to bend properly, or is he throwing his shoulder out in one direction? Does he try to do this at the trot? My own mare (very green) used to run me into the wall/fence all the time. The reason - one part of her body, namely her barrel or shoulder, was sticking out in that direction and I couldn't get it back in. Then, she didn't have the fine tuned body control to fix it. Now, she knows the cues to bring those body parts back into alignment but she will sometimes still have issues, mainly in the same direction your horse does as well. Why? Because she likes to drop/lean on her left shouder, and when she leans on her left shoulder while cantering to the right, we go right to the wall.
To correct it, look in the direction you are going, have an opening right rein, strong left leg driving to the right, and a balancing left rein. I often go back to work at the walk, trot, then canter and do spiral in's with her to the right, thinking about closing the circle down by using my outside aids to bring her in. You still keep the slight rightward bend, though. Then you spiral back out, driving them off the inside leg with a slight leg yield and concentraing on the bend, but keeping that balancing outside rein to prevent them from over bending. I have been known to ask for a leg yield at the canter is she is getting nasty about throwing that shoulder/barrel out to get them off that side.
Let me know if this doesn't make sense.