Rebuild the missing context
Assume the recipient will not remember every date, promise, or earlier conversation. For a message about how to apologize for choosing sides between friends, use this factual base: identify the moment, the impact on trust or inclusion, and what your friend should not have had to carry.
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 a repair that matches the harm and show how your behavior will be different. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Avoid making urgency into pressure
Do not use shared history, mutual friends, or public gestures to pressure a private response. If contact is unwelcome, stop and avoid using mutual friends as messengers. Do not make the other person responsible for reducing the discomfort that prompted you to write.