I have a 2 A 12 V regulated power supply and I want to use it to emulate a 9 V battery for a breadboard
If you want 9V to power the circuit, use a 9V regulator.  Or drop the excess voltage using a 2.7 or 3.3V Zener diode (reverse biased), or even five silicon rectifier diodes in series.

If you want to simulate the limited current capacity of a 9V battery, add a resistor of about 1.5 ohms in series with your power supply.  See this Stack Exchange question.

Any circuit that is broken by being connected by a power supply with a higher amps rating was probably a bad design in the first place.