Name the impact without exaggerating it
Describe who had to wait, worry, redo work, spend money, or make a choice with incomplete information. For a message about how to apologize for crossing a flirting boundary, use this factual base: name your behavior, its likely practical or emotional impact, and the agreement or trust it affected.
Make repair practical
Offer the repair you can actually complete, with ownership, timing, and any limit stated honestly. 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.
Let future behavior carry the proof
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. A promise inside a message is weaker than a visible change in what you do after it.