Voltage to current circuit simulation problem
Probably your gate is already saturated.

Try reduce R4.

You can't control the current accurately through R8 with this circuit, MOSFET have big temperature dependency and your voltage will vary as the MOSFET heats up. 

Also the gate threshold will vary from one mos to another. If you wan't any sort of stable current regulation you need some sort of feedback circuit.

You need to have some sort of feedback regulation.

Check this circuit that allows to control the current, can be a LED or Resistor.