The transformer should be 2 x 15 V AC and at least 20 VA
You may also use external heatsinks depending of if you really want to take out 500 mA continuously
The series resistors at the input should be 1 ohms or so, not the small 10 ohms.
Which voltage reference do you plan to use, LM431? I will calculate the feedback resistor values for you.
Set the reference to 6 volts (= 12 volts out)
R43 = 6.8 k
R45 = 4.7 k
This will give you approx. 12.2 Volts.
P3, R41 not used