Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to discuss a career sacrifice with your partner, use this factual base: name the current pattern, concrete impact, and decision that needs both partners' attention.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask for a time and format to discuss one issue, then identify any outside support that could help. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not threaten abandonment, use intimacy as leverage, or demand an answer during an unsafe or heated moment. Use specialist domestic abuse or emergency support when coercion, violence, stalking, or immediate danger is present. You do not need the other person to agree with a boundary for you to follow it consistently.