Write from facts rather than momentum
Pause long enough to remove accusations, assumptions, and details that belong to someone else. For a message about how to apologize for hiding a purchase from your partner, use this factual base: name your behavior, its likely practical or emotional impact, and the agreement or trust it affected.
Offer a proportionate next step
Match the size of the request to the relationship and to what the recipient can reasonably decide now. 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.
Protect privacy on both sides
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. Use the most private legitimate channel available and do not share the exchange to recruit support or pressure.