Before I respond to a Slack message, I write my reply in iA Writer. Before I reply to an email, I write it down in iA Writer. Before I compose a tweet, I write it down. In iA Write.
Regardless of the significance, I archive each potential message I send to another person. This is the designer in me apprehensive to share anything I don't think is passable. It's also part ownership of my words and part self-effacing education. iA Writer's interface is a hyper-focused canvas that removes all UI elements except for the cursor and your words. There are a few cool features like night and typewriter mode which nail down your focus even more than running stock.
I'm not personally attached to the app, however I haven't found another one that is so restrictive and readable (no-one should read or write in 12 point font). The only thing it's missing is a Hemingway extension.
Since taking my responses to a familiar canvas, I've been able to relax and really think about what the other person is actually saying. I have a bad habit of assuming the worst when it comes to reading other people's messages. I haven't figured out why that is, but this method of response has helped more than I thought it would. Some of my friends have told me to try imagining the other person being extremely positive as they send their message, but that didn't help much.
I used to write reactively and usually within seconds of the message. iA Writer has helped calm my introverted tendencies and highly recommend capturing yourself this way if you haven't tried it.