What Causes Persistent Anxiety Despite Taking Medication?

I am a 38 year old male diagnosed with GAD, obsessive thinking and sometimes have difficulty traveling due to/resulting in panic attacks. For almost 10 years now I have taken Klonpin 1mg 4 times a day, Wellbutrin xl 150mg in the morning, and mirtazpine 15mg at night. I have found this combination to be the most beneficial because it helps my anxiety with hardly any side effects. Even though I m doing well I still feel anxiety (sometimes) stopping me from moving forward in areas of my life. I am seeing a therapist and this has been helping. My GP prescribes my medication and he has told me in the past that if I want I could increase the mirtazpine to 30mg or I could leave it alone. Lately I been wondering if 30mg would make more sense. Although I know the you never know until you try saying trumps all, I thought a psychiatrist might have feedback to help me make a decision.