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 apologize for breaking a date plan, use this factual base: name your behavior, its likely practical or emotional impact, and the agreement or trust it affected.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a specific change the other person can observe and ask what repair would actually help. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not explain away the behavior, bargain for affection, or use gifts to skip accountability. Respect separation, no-contact, and safety plans, and use professional support when coercion or abuse is present. You do not need the other person to agree with a boundary for you to follow it consistently.