I'm truly sorry; there's no excuse for my negligence.
C2A straightforward apology acknowledging fault and the lack of justification.
With hindsight, I should have handled that differently.
C2Acknowledging a mistake in retrospect.
I understand my actions caused considerable distress, and for that, I am deeply sorry.
C2Acknowledging the negative impact of the mistake and expressing deep remorse.
Allow me to offer my unreserved apologies for the unforeseen complications.
C2Formal apology acknowledging unexpected negative consequences.