Mirtazapine and experiance

iv been taking for3-4 years now,it was a major help to me but now i feel very dopey sometimes with dizzieness, i do feel this is a side affect of the drug,i have tried cutting it down but when i do i experiance terrible,terrible headache,and no pain killer gets rid of it.i'v never been on the full dose as it knocked me out and felt sick.i only got on to half a tablet daily,i take a quater in the morning and i dont have to remind myself to take the next quater,by 1 or 2 oclock i start to get the headache. also when i try to leave out the afternoon quater i have been known to have sevear stomache cramps by evening.i feel i need to get of this!!!!!!