Great question. Panic disorder is a medical oddity because the research still has not pinpointed why it happens to some people. There seems to be some faulty wiring in the "switch" that turns on the sympathetic nervous system, thereby initiating the "fight or flight" response out of (as you said) nowhere.
Panic DISORDER comes about when the fear of having an attack begins to wear away and change a person's life - and it seems that you are about at that point right now. So, here's how treatment works:
Unfortunately, as of right now there is no way to prevent the attacks from ever happening again. The good news: they do lessen in frequency, duration, and intensity as the sufferers age. The bad news: you will probably never be completely without them. Therefore, treatment includes a benzodiazepine to take whenever you feel an attack coming on (Xanax is the most oft prescribed, although the tranxene is certainly in the family).
Talk therapy focuses intensely on "weathering" the attack without driving it into the red zone by panicking. It is tough (as you are aware) to NOT think about heart attacks and stroking out and dying in the midst of an attack, but the anxiety swelling in your mind actually exacerbates the attack and gives it staying power. Once you know you will not die, the next step is to wait it out, go through relaxation techniques (most therapists can teach these relatively quickly), and help the mind soothe. In fact, average latency for the actual attack is only in the 8-10 minute range - the remainder is held up by those intrusive thoughts that ratchet up your fear.
I know that answer is not necessarily comforting, but at least know that it will not always be this awful. With all that is going on in your life - and especially things that feel out of your control - you are more prone to the attacks. In fact, folks who tend to internalize their feelings (or "stuff" them) are particularly susceptible... think about shaking up a Coke bottle. Re-connect with your therapist if you had one, or find a good one to help you through this phase.
If you are satisfied with the response, please hit "Accept." That is the only way I receive credit for my answer. Thanks-