Constant Sum Voltage Divider With Thermistor/Temp Sensor
If you don't care too much about the PWM frequency changing when the thermistor changes its resistance with temperature you can just use a fixed resistor for one side of the pot and the thermistor for the other side of the pot - I believe I know the circuit: -



To the left of the wiper, replace with a small value of resistance such as 100 ohms. To the right of the wiper connect your thermistor but, be aware - double check this - I may have the placement back to front. If the fan motor gets faster as the wiper moves to the right then it's OK!

By the way - lose the 470uF capacitor and definitely have a decent diode across the motor.