hi scuz,
A common way to run the motor slower is pwm, pulse the motor with 12v square waves, such that the duty cycle is around 50%.
Its also usual to measure the back emf of the motor in order to adjust the pwm for the motor run at a constant speed.
If I understand correctly, you have two resistors in series with 12v across the resistors, so the centre point is at 6V , without the motor connected?
If so, as soon as you connect your motor in parallel with 'lower' resistor the voltage will drop at the centre point.
Eric