If drifting to the inside then possibly inside shoulder collapsing into the turn and quarters swinging out. Turn your body a bit more in direction of turn without losing contact on outside rein. Look where you are going. Turn shoulders which will place outside leg in correct position to support. Don't place too much weight into your inside stirrup.
Always have them focusing on YOU. Try using a crop and use it once in a while when he/she isn't listening. Always keep the outside rein a bit tighter than the inside rein but don't have the rein shorter just a bit tighter. This way the horse knows that he/she is supposed to go straight forward. Just try to keep their focus. I hope your horse gets better on this! :)