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 decline a business partnership proposal, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Give a direct no, a limited alternative only if you mean it, and any practical information they still need. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not overexplain private details, blame another person, or say maybe when the answer is no. When pressure, coercion, or retaliation is possible, use a safer channel and involve appropriate support. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.