As a dressage rider, I would not suggest using a whip on this horse, especially if the horse is already unclear on the concept of turning/steering correctly. You are going to complicate things and frustrate your horse.
Having someone stand in a corner sounds like a good idea. But its not. If the horse really does continue running into the corner with someone standing there rasing thier arms, this is going to encourage rearing. That is a serious problem.
The best advice is to find a good trainer who knows what they are doing. None of us have actually seen your horse. And it could be something as simple as ill fitting tack.