If they screenshot other people’s messages to show you, your messages are in someone else’s camera roll.

Somebody who shows you private conversations is not doing you a favor, they are showing you their method. The small thrill of being on the inside distracts from the obvious part, which is that you are just the current audience. Nothing about the arrangement changes when your turn comes. They will not even experience it as a betrayal.

When someone hands you a screenshot, notice the pull of feeling trusted and put it down. You do not have to make a scene, just stop giving them material. Write to that person as though anything you send could be read out loud.