Mate, been there. Your problems will not be solved by Mirtazapine alone given that you have obvious sleep anxiety. I implore you to buy and read the book The Sleep Book by Dr Guy Meadows: https://www.amazon.com/Sleep-Book-Well-Every-Night/dp/1499250533/ref=sr_1_3?ie=UTF8&qid=1548365652&sr=8-3&keywords=the+sleep+book
The brain WILL NOT switch off when you’re in an anxious state. It is biologically designed to keep you awake if you’re facing a threat (and you’ve associated sleeping with a threatening event). You need to decouple the idea of sleep from the idea of threat. This book will help you do this.
Do this in conjunction with the Mirtz and I think your sleeping will improve dramatically.