PWM motor control for keeping in constant speed for different loading

When motor applied different loading, the motor speed will also change if there are no control circuit applied. So, I would like to design a
control circuit for dc motor by using PWM. The target is that keep the motor into a constant speed even we applied different loading on it. But I have faced a lot of problems on design. So, anybodies can provide a control circuit to me to achieve the above task.
I am using a brushless DC motor on speed 3000rpm. And using a PWM circuit which feedback signal is from the hall sensor. By using the hall sensor signal (a pulse signal) to convert to DC voltage which is proportional to frequency of that pulse signal.
Really hope any guy can help me ~ Thank you very much~
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