If you stopped the mirtazapine without tapering the dose downwards, slowly, slowly, then you're going through a rebound effect and withdrawal.

In my own experience, I never had any withdrawal as severe as the one from mirtazapine. I seriously feared I'd never sleep again. Talk about messed up!

After three days, you could probably jump in at about half your old dose, and try to taper the dose down from there. It took me about three weeks, slowly reducing the dose, to get off the stuff. I broke a bunch of pills in halves and quarters, then sorted them on size. They never break evenly, so you can approximate the drug dose based on the size of the fragment.

Zoloft was much easier to get off, for me. The withdrawal was much easier, but you still have to taper the dose. Don't just stop taking it, all at once.

Good luck with it,
Lar