What Causes Extreme Depression And Insomnia Despite Taking Mirtazapine?

Hello, I have been diagnosed with ptsd about two years ago. I was prescribed 30mg of mirtazapine. I was not sleeping and experiencing suicidal thoughts. The mirtazapine helped for about the first six months which i think was because I could actually get 5 hrs sleep. In the last year or so I have experienced extreme depression and suicidal thought lack of energy, no solid sleep. Very sad all the time and have a hard time with coping skills. I have a hard time performing tasks and nothing seems to interest me anymore. Do you think i could be bipolar and my dr doesnt listen that i am back where i started. Anything to help?