I think you answered most of your questions. You obviously have an anxiety/stress problem. IMHO you possibly have some issues w/ hypochondria as well. You said you are in therapy. Surely some of these issues/possibilities have come up in therapy sessions?
Stop reading the med books. You're only going to scare yourself. I'm neither a doctor nor a therapist but it seems obvious that clonazepam works better for you than xanax. Clonazepam was the only anxiolytic that helped me. Buspar and Xanax did absolutely nothing for me.
If it gives you any comfort, I went for weeks/months w/ a bpm of over 200 during a bout w/ anxiety. That was about 20 years ago. My heart is still ticking just fine
For your peace of mind have a thorough checkup....better safe than sorry. Then concentrate on the real problems.
Good luck.