I'm not a qualified doctor, but it sounds exactly like generalized anxiety- something I battled years ago and beat it.
My suggestion is read some books on anxiety and check your diet. This is no joke.. What you eat is an enormous trigger for this sort of thing and getting a hold on your diet/health will give you a sense of control over your life that nothing else can do. I had to cut out all caffeine, cut back on sugar, be mindful of not eating too close to bedtime, and not trying to fall asleep on the left side (as it puts more pressure on the heart and you can hear it easier - sleep on your right side if you can).
Primarily I found the biggest issue to be a loss of a sense of control over my body, which lead to me feeling trapped in my own body... Not a good feeling- it sent me to the ER a couple of times with the heart thing and dead brain exactly as you mentioned.
Don't give up, and do understand that
you CANNOT die from having anxiety. Your body is not a timebomb, it will act and react accordingly to keep you alive and ticking- you can COUNT on that! So rest easy.
This is all easier said than done, which is why I suggest you go to a library and take out a few books about anxiety- I have a few I can suggest if you are interested.
Panic attacks debilitated me, but it is curable- I am living proof

We are here for you, and especially I will be here for you. You will have to relearn how to trust your body. It is likely that no doctor or medication will be able to help you, the best way to do it is behaviorally. Time to take control of your life and feel better!