Rebuild the missing context
Assume the recipient will not remember every date, promise, or earlier conversation. For a message about how to apologize for forgetting to return a call, use this factual base: name the exact action, the inconvenience it caused, and what you understand now.
Say what needs to happen next
State the decision, information, or practical action you need and include a real deadline only when one exists. 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.
Avoid making urgency into pressure
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. Do not make the other person responsible for reducing the discomfort that prompted you to write.