Regulating D.C. motor - Feedback
In order to get a feedback of something, you need a sensor for that something. To get a position or speed of a motor, people usually use some kind of position encoder (optical, magnetic or maybe some other technology I am not aware of). Your motor is obviously lacking such a sensor. So your option would be spending more on your hardware and buy a motor with an integrated encoder. Or install a position sensor elsewhere on your system, such that you can measure the motor speed/position indirectly.