You need a Pulse Width Modulated Motor Driver, consisting of an large MOSFET transistor mounted on a heatsink, driven by a variable pulse width multivibrator (pulse generator), such as
this one.
The MOSFET acts as a switch, which has a very low (0.1Ω) resistance when ON; and a very high (almost infinite) resistance when OFF. The 555 timer makes the variable OnTime/Off Time while keeping the pulse rate more or less constant, but at a rate so high that the motor responds to the average current through it, making a smooth speed control as the pot in the circuit is turned, and providing lots of torque when starting.
Trying to speed-control by putting a resistance (
rehostat) in series with the motor is an extremely crude and wasteful way of doing it. At some settings, the rehostat will get stinking hot (wasting energy) from your battery. The speed regulation sucks; the slightest change in mechanical load on the motor will cause the speed to change; while with the PWM method, the motor speed will stay much more constant.
To crudely speed control your motor, it would take a huge 300W 10Ω wire-wound Rehostat with a 6A current rating, like the MP40 shown in
this document. Unless you can find one surplus, you will find that the cost of this rheostat is much higher than the simple MOSFET speed control....