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。

It doesn't matter. Please don't blame yourself. 没关系，别自责了。

It doesn't matter. 没关系。

I'm sorry I forget to bring you the book. 很抱歉我忘了给你带书了。

It doesn't matter. 没关系。

I'll bring it next time I see you. 下次见你时我给你带上。

OK. 好的。