The heat is normal and expected. You have chosen to use a resistive type devce (resistor) to "scrub" of the extra voltage. It does this by converting the voltage into heat, P=IxE or watts(heat)= current x voltage. The other way is to use semiconductors or transistors with a simple circuit. The transistors have to be somewhat large (expensive) to handle the the current but the heat is not there.

Six volt guy living in a twelve volt world