Your Dr would have prescribed Mirtazapine after taking into many considerations, your general health, lifestyle, side effects to the medication etc. That's why there are so many different types of anti depressants. You need to give your medication 4-5 weeks to see the " therapeutic " benefits. If by then, there are no positive effects, go back for a chat with your GP.
Some anti depressants oddly can cause the user to feel more suicidal. For the benefit of everyone reading this post, the reasons anti depressant use des have " risks ", is this. Someone may not even have the motivation to act on their distressing thoughts - suicidal - they become so down they just want to hide away and sleep a lot. The danger comes is when they take the anti depressant, it lifts their mood, but, lifts their mood to the extent that they now have the motivation " drive " to take action on their suicidal ideation.
Hope that helps Noble, take care.