Initial disclaimers as to writers usual reasonableness. Claim of exceptional situation provoking previously unknown emotions. Vague reference to possibility of being unreasonable.
Description of writer in calm, reasonable mood being provoked with no cause into incandescent rage. Detailed explanation of how other party is thoroughly at fault. Passing reference to own feeling of guilt / contrition even though writer has acted very reasonably at every turn.
Express desire for other party to suffer consequences, preferably severe. Appeal for sympathy and agreement that writer is thoroughly reasonable. Emoticon.