Have you had your heart checked out to make sure everything is okay? If so, then it may be that the Xanax is not enough for you. Call your doctor and let him know that it's not helping you. He will either up your dosage or switch medications. Always work closely with your doctor, he needs to know when the medication isn't working, or you have a new symptom.
I would point you to my post to Dwood, I think it the poster.