As a simple modification for a fairly low current load, you could add an NPN darlington transistor as an emitter follower after your potentiometer circuit, to give it higher current capability.
The pot wiper would connect to the base, +10V to the collector and the emitter connects to motor positive.
Also connect a diode such as a 1N4000 series (4002, 4004, 4007 etc) across the motor output from the circuit, with cathode (band) to the transistor emitter and anode to 0V. That protects the transistor from any inductive spikes (back EMF) that the motor may produce).
The output voltage will be around 1.2V - 1.5V lower than the pot voltage, so you may need to compensate with the supply voltage.
The transistor will get warm (or hot) and needs a heatsink.
A TIP120 is readily available NPN darlington which should be suitable.
That pretty much gives you the same circuit as used in many simple model train speed controllers - eg.
That has an AC power supply included and a reversing switch in the motor connections - but the actual "speed control" part is a potentiometer and darlington transistor...