Sorry to hear that. I know the feeling, its the unknown...
I go into afib if I get in the 150bpm range. So I keep it down if I am going to be stressed. Often I will take 10 mgs of Propranolol as a 'governor' before a workout or hike. It seems to work for me as it keeps my rate lower enabling me to get a good, moderate workout. I don't seem to have any untoward effects from it. But, I imagine you already know about beta blockers.
Hang in there, we can all relate.
Jeff