Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to apologize for making a decision without your partner, use this factual base: name your behavior, its likely practical or emotional impact, and the agreement or trust it affected.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. 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.
Respect the boundary already visible
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 block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.