Typing a furious reply and deleting it before you hit send is a small win. Count it.

Writing the furious version is useful, and sending it is the only part that costs anything. The typing discharges most of the heat, and the delete gets made by the version of you that still has judgment. Nobody sees a win like that, which is why it is easy to dismiss, but it is the same muscle as every other kind of restraint and it builds up.

Start counting them. When you catch yourself drafting something hot, finish it in your notes instead of the reply box and put the phone down for ten minutes. If it still needs saying tomorrow, it will say itself better.