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 bringing an uninvited guest, use this factual base: confirm the booking, timing, guest, transport, or budget change and who now has to adjust.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. Offer a replacement plan, reimbursement, or clear decision before the next real deadline. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respect the boundary already visible
Do not minimize lost money or time, and do not invite extra people into a plan without consent. Follow venue, transport, insurance, and local safety rules when an emergency or stranded traveler is involved. A block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.