You may be suffering from treatment resistant OCD with comorbid Anxiety Disorder and treatment with Mirtazapine alone may not do much good. You may need at least 2-3 drugs along with Cognitive Behaviour Therapy (a type of Psychotherapy) to see improvement. You must also know that the treatment of OCD takes the longest to show appreciable effects, even up to 12 weeks so I would suggest you to get in touch with your Psychiatrist, explore viable treatment options and start therapy and remain in therapy as long as you do not get symptom relief.
Next Steps
Visit your Psychiatrist and tell him/her the problems you're facing