I am writing to apologize for the unsatisfactory work I have done for you. I understand that the quality of my work was not up to the standard that you expected and for that, I am deeply sorry.
I take full responsibility for the mistakes I have made and I understand that I have let you down. I realize that I should have taken more time to ensure that the work was of the highest quality and I regret not doing so.
I understand that this is not the first time I have let you down and I want to assure you that it will not happen again. I am committed to improving the quality of my work and I am willing to do whatever it takes to make sure that it meets your expectations.
I am aware that this mistake has caused you a great deal of inconvenience and I apologize for that. I want to make it up to you by offering to do whatever I can to help make things right. I am willing to work extra hours or take on additional tasks to make up for the mistake I have made.
Once again, I apologize for the unsatisfactory work I have done and I hope that you can forgive me. I am committed to doing better in the future and I will do whatever it takes to make sure that I meet your expectations.