句：Please don't blame yourself.

别自责了。

A: I'm terribly sorry that I made such a mistake.

我对自己犯这样的错误感到十分抱歉。

B: It doesn't matter. Please don't blame yourself.

没关系，别自责了。

第二句：It doesn't matter.

没关系。

A: I'm sorry I forget to bring you the book.

很抱歉我忘了给你带书了。

B: It doesn't matter.

没关系。

A: I'll bring it next time I see you.

下次见你时我给你带上。

B: OK.

好的。

其他表达法：

I quite understand.

我很理解。

This can happen to the best of us.

这种事谁难免。

It't not your fault.

这不是你的错。