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I'm sorry; (it's) inexcusable; I feel regretful; I feel guilty
thank you very much (for help, etc.)
ご迷惑をおかけして申し訳無いと思います。
I feel sorry for the trouble I have caused you.
お待たせして誠に申し訳無いことでした。
I am truly sorry for keeping you waiting.
期限に間に合わず申し訳無い気持ちでいっぱいだ。
I feel deeply sorry for not making the deadline.