Hello user
I have read your statement and i understand your concern
It is important for you to understand that antidepressant drung like 
Cymbalta cause serious reactions when taken when 
Mirtazapine. Let me tell you again that reaction is rare but serious. Its called 
serotonin syndrome which can exacerbate your condition causing further confusion, 
hallucinations and irritability.
 If u are taking Mirtazapine for anxiety or 
depression, Change it to SSRI. If i were your Doctor i would have prescribed you Escitalopram 10mg daily for 6 months with short anxiolytic course of Alprazolam 0.25mg daily at night for 8 days.
Your headache and lack of sleep is due to severe anxiety. If mirtazapine is not working, your physician may change the prescription. Plus you can use short course of anxiolytic.
For short term management of your issue, you are advised to take Tab Dormicum 7.5mg 10 mins before going to sleep. However it should be noted that Dormicum should not be continued for more than 7 days\
In Nutshell,
Go to your physician and tell him your story, he will change your medication to a new and more effective antidepressent
Thanks