Dear [Name],
I am writing to apologize for my carelessness and lack of attention to detail in [describe situation]. I understand that my mistake had a negative impact on [describe impact], and I take full responsibility for my actions.
I understand that my carelessness has caused you [list consequences], and I am truly sorry for the inconvenience I have caused. I understand that this mistake was unacceptable, and I am deeply sorry for my negligence.
I want to assure you that I have learned from my mistake and will take extra care in the future. I have taken the necessary steps to ensure that this kind of mistake will not happen again. I have [list steps taken], and I am confident that I will be more careful in the future.
I understand that my mistake has caused you a lot of frustration and inconvenience, and I apologize for that. I am willing to do whatever it takes to make it up to you. I am available to discuss this further and answer any questions you may have.
Once again, I apologize for my carelessness and lack of attention to detail. I understand that this mistake was unacceptable, and I am truly sorry for the inconvenience I have caused.
Sincerely,
[Your Name]