15mg mirtazapine not working for insomnia : insomnia

I started taking 15mg mirtazapine 2 weeks ago for anxiety, insomnia and depression. My doctor specifically said it would help me sleep. I tapered down the benzos I was taking as I started mirtazapine and here I am, on a very stressful day, still up and completely unable to sleep and terribly worried about tomorrow. Have had to take some more drops and it feels like a defeat. If mirtazapine doesn't help me sleep is it safe to say it's maybe not the medicine for me? I'm new to antidepressants and I read that mirtazapine helps with insomnia straightaway but it's not the case with me. Any advice welcome. Thanks