1) I am awfully/terribly sorry for what I have done.

　　2) I feel very guilty for what I have done to you.

　　3) I am afraid what I have done has caused many inconveniences to you.

　　4) I regret to inform. you that I am unable to do…

　　5) Please accept my most cordial and humblest apologies for…once more.

　　6) I will try my utmost not to make such a stupid mistake again.

　　7) I am so sorry to have put you to so much trouble