1. I'm sorry for what I did.
2. Please accept my sincere apology.
3. I feel terrible about what happened and I apologize for it.
4. Please forgive me for my mistake.
5. It was wrong of me and I apologize for it sincerely.
6. My behavior was unacceptable and I am deeply sorry for it.
7. There is no excuse for my actions and I apologize unreservedly for them.