Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to set a boundary about a mutual ex, use this factual base: name the recurring contact, request, topic, or behavior and the effect it has on you.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. State the limit, any acceptable alternative, and what you will do if the pattern continues. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not diagnose your friend, list every past grievance, or threaten public embarrassment. For stalking, threats, coercion, or self-harm concerns, preserve evidence and use appropriate crisis or safety support. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.