In grad school and for a few years afterwards, I was afraid of opening e-mails (and other messages). It sounds like hyperbole to call it an actual fear, but I still have dozens of unopened e-mails from that era, even ones I know for certain contain nothing negative - it's not really about negative e-mail vibes, though that must be what sent this problem into overdrive, as the total number of negative or even mildly not-nice messages I've ever received is exceedingly small.
I stopped reading reddit because I frequently felt compelled to post replies but was taking extreme measures to avoid seeing what people replied to me (probably hundreds of unread replies there).
boingboing was always my sanctuary, but for a long time I didn't look if someone replied to me, and eventually greatly withdrew from here too. I do now make it a point to read any replies I get here when I occasionally do post something.
I'm a very reserved person in real life, but used to be really rather open and talkative on the internet - a classic tale. Except now I'm totally withdrawn from the internet too.
Conclusion: don't go to grad school...?
That does get me in an adrenaline-fueled fear state occasionally, but usually it's a milder mental block type thing.
What does always give me that physical fear is that creepy feeling when an area is obscured - either by darkness, or e.g. a dense forest (or a dense forest in darkness...) - and while you know 99.9999% of the time there's nothing there, there could be something there. I can't turn my back to something like that without an intense instinctual urge to run away, which I occasionally have done even though I know that just makes it worse.