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 leaving a mess behind, use this factual base: name the exact action, the inconvenience it caused, and what you understand now.
Make repair practical
Offer the repair you can actually complete, with ownership, timing, and any limit stated honestly. Offer the smallest useful repair and give a realistic time for completing it. 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 inflate the apology, fish for reassurance, or demand immediate forgiveness. If the person has asked for no contact, repair what you can without sending another message. A promise inside a message is weaker than a visible change in what you do after it.