Dear [Recipient],
I am writing to express my sincerest apologies for not fulfilling the promises I made to you. I understand that this must have been very disappointing and frustrating for you, and I am deeply sorry for letting you down.
I take full responsibility for my actions and I understand that there is no excuse for not following through with my commitments. I understand that my behavior has caused you to question my reliability and trustworthiness, and I want to assure you that this was not my intention.
I understand that I have let you down and that this has caused you to feel betrayed and disappointed. I want to assure you that I am taking this matter seriously and that I am doing everything I can to make sure that this does not happen again.
I understand that words alone cannot make up for my mistake and that I must take action to demonstrate my commitment to honoring my promises. To that end, I am willing to do whatever it takes to make it up to you. Please let me know what I can do to make this right.
Once again, I am truly sorry for not fulfilling my promises. I understand that this has caused you to feel let down and betrayed, and I want to assure you that I am taking this matter seriously and that I am doing everything I can to make sure that this does not happen again.
Sincerely,
[Your Name]