The chances of you dying from panic and heart flutters is very, very slim and irrational thoughts like that make you panic more, which makes you worry you're going to die even more, it's a catch 22. If you find yourself thinking irrationally like that, stop for a moment, breathe calmly and tell yourself that you're being irrational, it's just your mind acting up and it's SA or generalised anxiety that's making you feel this way. The more you refuse to accept and fight against your negative thought patterns the easier it will become. I used to have very severe SA, I couldn't even leave the house, but by challenging my thought patterns and realising that it was the way I was thinking that fuelled my SA I improved an awful lot.
I've never tried beta blockers but I've heard they're pretty good, I hope they help x