Make room for the other version
Ask what the other person understood before assuming that you are arguing about the same facts. For a message about how to apologize for not defending your partner, use this factual base: name your behavior, its likely practical or emotional impact, and the agreement or trust it affected.
Correct the record precisely
Correct only what you can support, acknowledge any real impact, and propose a practical way to prevent repetition. 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.
Agree on a testable next step
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. Do not recruit an audience, publish private evidence, or turn a misunderstanding into a campaign about character.