Dear [Name],
I am writing to express my sincerest apologies for not fulfilling the promise I made to you. I understand that I have let you down and that my actions have caused you distress and disappointment.
I take full responsibility for my actions and can only imagine the frustration and hurt that you must be feeling. I am deeply sorry for not being able to keep my word and for not being able to deliver on my promise.
I know that this is not an excuse, but I want to explain the circumstances that led to me not being able to fulfill my promise. [Explain the circumstances]. I understand that this does not excuse my behavior, but I wanted to provide some context for my actions.
I understand that this does not make up for the fact that I did not fulfill my promise, but I want to make it up to you in any way I can. I am willing to do whatever it takes to make it up to you and to regain your trust.
I understand that this apology may not be enough to make up for the hurt and disappointment I have caused you, but I want to assure you that I am truly sorry for not being able to fulfill my promise. I will do whatever it takes to make it up to you and to regain your trust.
Once again, I apologize for not fulfilling my promise and for causing you distress and disappointment. I hope that you can find it in your heart to forgive me and that you can accept my apology.
Sincerely,
[Your Name]