Mirtazapine has an anxiolytic and sedative properties - as does xanax. Curious that you would be asked to taper both of them at the same time, and to make such a huge drop? I would think that tapering from just one at a time would be hard enough, but I'm not your doctor . But then, my doctor told me to just quit long term xanax and ambien usage in three weeks. I didn't know better, neither did he, and my quitting from long term usage in just a few short weeks was like someone opening a door to hell, shoving me through, and slamming it behind me. Doctors don't always get it right, and sadly sometimes they really get it wrong.
Paxiled is right - everyone is different, and we all recover at different rates. You took a low dosage for a month, you should be able to quit xanax very quickly with minimal or no side effects at all.
I'm not a doctor and I don't know you or your needs, so don't take what I say as medical advice. I can say, however, as someone who has been there and done that, that if you don't really need the xanax, then get off of it as fast as you possibly can. These drugs really do a fantastic job of taking the anxiety away - I wish I could take xanax every day of my life, it's a wonderful thing to not have any anxiety, to just feel normal. But alas, it doesn't last, and there is an extremely severe price to pay for the temporary relief it gives us. Quit while you can.
The right taper schedule is whatever works for you -- it's not going to be the one that works for the guy sitting next to you. If your doc's schedule works fine, great. If you think it's too fast for you, tell him to slow down and take it as slowly as you need to -- that's the only correct tapering schedule. But the above post is right, the less time you're on these meds usually the easier it is to stop taking them, so don't expect the worst, but do be prepared -- brains are just very individual things and we don't process these drugs the same way.
Since you have only taken it for one month, and since you are not taking that much, tapering off of it should be relatively easy. I took it for a couple of months, and spent just a few weeks tapering off of it, and had no problems. Try your doctor's taper schedule and see how it works. Also give some thought to why you took it in the first place. If it is a chronic condition, you might still be experiencing whatever caused you to take start taking it. Xanax won't make that go away, it just numbs you for a while.
Google The Ashton Manual. Read through it. Most of that is for people who have been taking much higher dosages for a long time, but you will still find some good information in it.