It's possible your doctor started you off at too high a dose. The normal starting dose for mirtazapine
is 15mg and titrating up to a maximum dose of 45mg.
Since it seems to be very sedating, you might want to discuss lowering the dose with your doctor, which you have every right to do! If he/she insists the sedation will pass, as the patient, you still hold the right to request your dose be lowered. If the 15mg doesn't help you after a months trial, you can always move the dose up.