Chen,
Yes, I have also been down the road of being on something for depression a long time back. Took me a long time to "wean" myself off it, very gradually is a must. I have avoided any of these for a long time, and have just preferred mainly using different holistic approaches to POIS symptom reduction and keeping physically as well as possible.
As Daveman emphasises, don't stop suddenly. Try find a GP who will work with you on a more general level, rather than just a tablet approach. You MAY benefit from one of these types of drugs, as others do. However, they usual work best, when combined with some other strategy, be it counselling or physical exercise or a wellness strategy etc.
My heart goes out to you man, as you are obviously younger than I, so it is more difficult to reduce the need for regular masturbation. I have been there. It is shocking in truth that avoidance of any sexual activities can be a help to a POIS sufferer.
Your reactions to Mirtazapine may indicate this is not a drug for you. Please get good balanced medical advice and keep on a wellness track. As GP's don't know POIS, best stick to what happens to you, rather putting a label on yourself, when they only believe in anything with medical validation studies behind it. On the way it appears.
Is it also feasible that it is something other than POIS that is equally at play here for you. Please don't give up on yourself or the possibility of being well some day in the not too distant future. A lot is happening out there with POIS research and other things to try.
I am happy that I have not given up, as there were many times I felt like it. Had I done so, I would not have found new hope, learned out about POIS and that others had this, and that something is being done by a seriously committed bunch of guys and the dedicated work of the Moderator here.