What opioid you took is gonna make a difference.
The halflife determines the length of withdrawals.
And mirtazapine isnt gonna help much at all, mean it could make you tired at night and you might get some sleep easier. But it will not help stop the actual withdrawals cuz they hit totally different receptors. It could be considered a comfort med to a degree
id get some Loperamide tho maybe take 10-16mgs of that a day and it would take the edge off quite a bit. It works off opioid receptors but doesnt get you high just helps kill the physical sypmtoms quite a bit. Its available over the counter, its like a poor mans methadone.
Taper it down over a week 2mg drop per day and stop.