Horse that drifts out/in

It sounds as though the horse is falling out through the left shoulder. This can have several different root causes, though. Often when this happens, the horse has dropped or rotated its barrel to the left. It is common to get stuck pulling on the right rein to compensate. Start by sitting as straight over the horse as you can, without collapsing at the waist on either side. Then focus on lining the shoulder up with the hindquarters. In other words, on the right rein, move the horse off your outside aids and bring the shoulder in, and on the left rein, focus on moving the shoulder out. Use as little rein as possible for this. This helps the horse straighten its barrel and carry itself more evenly. It can take time and schooling, though.

The very best of luck 🌸