It is quite common to have a reaction like this to heart surgery..its penetration into the deepest part of the body/mind/trust system... It takes time to get over it, and you will be the same again, but you do have to work on it. Expressing your fears to your therapist will help...don't hesitate to say what you fear, even if it sounds irrational.
Since depression is not the issue, I would ask your doctor about antianxiety drugs like klonopin or xanax that can be used occassionally only when the anxiety gets too bad.
Anyway, there are several other antidepressants that might work for you. Dont give up. Talk to you doctor again.