ToothFairy

I have an analog input which measures a fluid pressure in pipe. I want to control the pump speed to make sure the pressure is held in preset value. What would be the best way to do this? I tried to do a simple program which did a simple add/subtract to frequency output value depending whether the measured pressure value were higher or lower than the set value. This works, but it is very slow. Also the response time this way is way too slow. For example, if I close a valve in the output pipe, it takes a lot of time to adjust the pump seed low enough. Is there some standard way to do this? So I don't need to invent the wheel again..