I'm very sorry.
It has been a long time since I last contacted you.
I have not been in touch with you for a while.
大変申し訳ありませんでした。
I really don't know how to apologize to you.
無礼です。
I'm rude.
I'm very sorry.
I'm very sorry.
I'm very sorry.
I'm very sorry about that.
大変申し訳ありませんでした。
Game over.