Mom asks an important question, which is, is this one month the only time you've suffered anxiety? If so, I'd have to ask your practitioner, why not try therapy rather than go immediately to much more invasive and side effect laden medication? Was there something that happened a month ago that triggered this, and if so, is it something you might work on in therapy rather than get immediately stuck in the drug merry-go-round? But assuming going on medication was a good idea, start-up side effects, while very different for different people, can be pretty profound, and they start as soon as you start taking the drug while the effects you're actually looking for, a reduction in anxiety, if they're going to come at all won't be felt for a few weeks. What you might be feeling is just sedation from starting the drug, or it can be a lasting side effect of the drug that isn't going to go away and might indicate this isn't the drug for you. Sometimes it takes more than one try to find one that both works and is well tolerated, but it takes some time to learn that. Now, what you describe can also be a symptom of a host of things that might indicate you don't have an anxiety problem at all but a physiological one, something that always should be ruled out before giving someone an antidepressant. Some problems that can cause both anxiety and what you're feeling are thyroid problems, problems with B vitamin absorption, a poor diet lacking electrolytes or certain minerals or vitamins such as D, blood sugar imbalances, hormonal imbalances, sleep disorders -- there's a lot that can mimic mental illness and can cause the fatigue and lack of motivation you're feeling. It can also be depression. We all do better with a proper diagnosis and proper treatment, but that can be hard to come by sometimes.
Did you identify what causes your anxiety or try therapy? How old are you, and is this one month the only time you were anxious?
You would have to sit down with the person who prescribed this med to figure out if it is the cause of your feeling of brain dead. Side effects vary among users, plus people in a high anxiety state can easily feel there are effects that don't really exist because you feel so bad that it is hard to concentrate.