You could use a shunt feedback resistor as KlausST suggested, but add a single-supply type opamp (such as an LM324, LM358) with non-inverting gain to reduce the value of the shunt resistance and its dissipation.
For example a gain of +6 will reduce the value of the needed shunt resistor to 50mΩ and its dissipation to 0.2W for a 2A constant-current output.
You likely will have to add some compensation capacitance across the opamp feedback resistor for stability.