"bonus points for not answering 'go outside drink water read a book' etc etc'

Erm, well, the very first thing I suggest is going outside for a bit. A walk can do wonders.

But this is one part of a general strategy, really - when you are upset by something online, make a little space between yourself and the conversation, reflect on why it upsets you, and decide if it's worth continuing to engage.

If it is, try to be as even-handed in your responses as possible. I find deliberately trying to tone down my own rhetoric makes me feel better - if I'm using super emotional language, all I'm doing is ratcheting up my own pissed-off-o-meter. I have a better experience trying to discuss something in good faith with a fellow human, even if they're wrong or just being an asshole for the sake of it. Try to make a space for the other human to have a real discussion. Either it will become a good conversation, and might expose you or the other person to viewpoints you haven't considered, or it will become clear that it's not worth your time to continue.

If it's not worth your time (either upon initial reflection or upon fresh evidence), just stop. Nothing good will come of continuing to go at it. Look at or do stuff that carries value for yourself instead.