Is it hyperthyroid?
If hyperthyroid, your anxiety
and possibly the heart
sensitivity are due to this underlying condition.
Aggressively treating the thyroid should alleviate the anxiety - eventually.
In the mean time there are other anxiolytics, a wide array. One can certainly be found different from what you initially tried - and yet be very effective and well-tolerated.
Assumes that heart rhythm simple tachycardia.