Justin, your motor has to be defective. The circuit always provides a full 12V to the motor for high speed. 12V should NEVER make it run faster than intended. The only way that can happen is if the motor windings have broken down and it is drawing more current than it should. Adding resistence in series is only a bandaid. It will continue to breakdown until it fails completely. It would be best to fix it correctly sooner rather than later. As is said, it is better to do it right than to do it twice.