I agree with Mr Andy, you need to give it more time Phil.
Don't read too much about withdrawal horror stories, as everyone is different. Mirtazapine is one of those meds that needs to build up in your system,so you would be best to wait and see for a few more weeks.
Also, you could always try to add on pregabalin in a few weeks if you need a boost. Like a previous post says,they go well together,so it is worth considering.
I know how you feel though. I too need my meds to work,and it sometimes feels like not much is improving. I have always been impatient, and my anxiety runs riot when I think how much better I have felt in the past.
Don't give up yet Phil,lets keep fighting this together.
Loreen x