1. I hope you can accept my apologies.Please allow me to say sorry again.
2. Once again, I am sorry for any inconvenience caused.
3. I would be very grateful if you could tell me if there is any way that I can make up for it.
4. I do apologize for any inconvenience this may cause you.
5. I hope you can pardon me for this matter again.
6. I am sorry for causing you inconvenience.
7. To be frank, I completely forgot about it until it was too late, I am very ashamed of myself, and hope you will forgive me.
8. Would you please excuse my neglect in returning your book?
9. I trust you will overlook my mistake, which I terribly regret.
10. I sincerely hope that you will be able to think in my position and accept my apology.
11. I am terribly sorry that I had to cancel our appointment yesterday.
12. I am very sorry indeed to have to refuse your request as the thing is quite beyond my power to do.
13. I am awfully/terribly sorry for what I have done.
14. I feel very guilty for what I have done to you.
15. I am afraid what I have done has caused many inconveniences to you.
16. Please accept my most cordial and humblest apologies for…once more.
17. I will try my utmost not to make such a stupid mistake again.
18. I am so sorry to have put you to so much trouble.