Hello Grace (what a beautiful name)
I have a long history of mental health problems (i have bipolar 1 but for a long time was only treated for the depressions before being correctly rediagnosed). So I have been on pretty much every anti-depressant available, and I have experienced bad dreams, nightmares, weird dreams and eidetic imagery (vivid imagery playing in front of your eyes when you close your eyes to try to sleep) - with virtually all of them. Unfortunately its a sign that they are working! - it means stuff is happening inside your brain that will eventually make you better.
However I coimpletely sympathise with how distressing they can be. Waking up with a jolt at 3am and having to put the lights, radio, telly anything on to try to banish the horrible feelings of terror, surreality, horror etc... its no fun.
I keep a bedside lamp within very easy reach and a soothing book by the side of the bed (something really calm and easy, like a cookbook or a gardening book or anything with pictures like a 'coffee table' art book) to lull you off to sleep with nice images in your mind, and to pick up if you wake in the night.
Also be kind to yourself whilst waiting for the mirtazipine to work which can take up to 8 weeks - and you will notice the emotional improvement last of all. Then you will look back and realise, actually from about 4 weeks onward things like appetite, work, relationships were getting a little bit easier even if you didnt start to feel happier in yourself til about 8 weeks of treatment.
Mirtazipine saved my life too. I was in severe, suicidal depression with schizoid features (I was completely withdrawn, wouldnt leave the house, had hallucinations and delusiona thinking). Within 5 weeks the improvement was dramatic. I fully beleive if I had not taken mirtazipine i would have taken my own life. So your Doc has realy given you a good medicine that really does help.
15mg is a nice safe low starting dose - but mirtazipine is also a bit unusual in that it is more sedating at lower doses. So the doctor is trying to help your sleep by giving you an anti-D that helps you have deeper sleep and should therefore lessen the number of nightmares you have. I certainly found I was much, much better with mirtazipine than the bizarre dreams I got with other meds!! As your sleep patterns get better the number and severity of bad dreams will dramatically improve.
I really hope you have the same experience as myself and so many others find with Mirtazipine: that your doctor prescribed it is a very good sign that they are taking you seriously. Now you just need to take it easy - slow down, and just make sure you get plenty of rest as well as sleep, eat well, bit of fresh air and exercise if you are up to it ... Reallly take it easy on social activities for now as they can occasion stressful interactions which may not be helpful. You're ill - for now, all you are expected to do is look after yourself and take that pill every night without fail!
All the best for a good recovery x