I've had an anxiety attack last well over a week (this past May) I've had them last anywhere from 12 hours to two weeks. I have panic attacks that last from 30 seconds to 2 hours.
I wouldn't play around with your heart though, it could just be that Ativan doesn't work for you, perhaps something else will. It could be anxiety, but then again it could be something physical. Be sure to make an appointment with your cardioligist.
Anxiety attacks can cause similar symptoms, but anxiety attacks aren't the only thing that can cause these symptoms.
I hope you find some help and some answers