It could be a combination of taking things too personally + not picking the best friends. Like your externship example sounded more like taking a comment in a way it wasn't meant... but it also sounds like your friends are not there for you the same way you are there for them.

I came to see a few years ago that I was really bad at picking out friends. My friends were kind of like yours, going on and on about their problems but never interested in what was going on in my life. After a few years of that, I would eventually get to a point where I couldn't take it anymore and break off the friendship (kinda like your boiling over). I started to see that this had been a pattern throughout my life and now try to be more careful about who my friends are.

I don't know how you stop taking things personally because I struggle with that too. I do the same tricks, trying to talk myself out of it, assume that the other person has the best of intentions, but it's really hard. Sometimes it seems easier to just stop myself from even thinking about it at all, but that's also easier said than done.