Regulating power on load
The only way to control the current is to control the voltage.  If you don't want to do this digitally, and your FET can handle some dissipation, put in a current sense resistor and differentially measure the current through the resistor and the voltage across the load.  An analog multiplier will take these readings and output power.  Compare your power reading to a reference designating the desired power with an op amp (with the reference on the positive input), and use the output from that amp to drive the FET gate.  Make sure the driver stage is slow enough to prevent instabilities from delays in the other components, particularly the multiplier.