Recognize what makes this moment difficult
Acknowledge the event directly without turning it into a lesson, comparison, prediction, or silver lining. For a message about how to decline an invitation to be in a wedding party, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Offer support that can be accepted
Offer one specific form of practical or emotional support and make clear that no immediate reply is needed. 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.
Follow their pace rather than yours
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. The recipient decides whether they want conversation, ordinary company, practical help, professional support, or quiet.