Follower current source?
Maybe a current mirror, depending on how much current you need.  A current gain of 1 is pretty reasonable if you just get two of the same FET to use in the mirror; although depending on tolerance it could vary between .5 and 2-ish.  You could use power MOSFET's in the mirror.
current-mirror-wiki

Example schematic below:
It is for positive voltage, but topology should work if you change out PMOS and VDD/GND for GND/VSS
EDIT: I am trying to interpret your request.  But if you are trying to control the current using some 0-20V source, or a knob, or a combination of the two, this may work for you?  Also assuming you want the 300V AC signal to be the supply to the load.  Because yes, in this case when the AC sine voltage decreases to 0V you will not have any voltage across R3.  But in this case you need to use an AC-DC converter.







simulate this circuit &ndash; Schematic created using CircuitLab