A sincere apology letter is one that serves to tell it like it is, avoiding the lies, deceit and excuses that may tempt the writer to turn into a storyteller. Maintaining an aspect of truth and sincerity has to come from a deep feeling of regret and sympathy that allows you to pen off your true emotions and explain that you require the other person to understand and empathize with you.
This requires you to sit down and replay the whole situation in the head and weigh the advantages of using the truth over lies and also helps you to choose the right words after making a draft letter that drains all your emotions into the paper. Once you have the choice of words, you are at liberty to select those that both portray the aspect of guilt and submission and also seek to be pardoned and aim at reconciliation. It takes skills and a willing heart but once you are in the right mood and feel the need to reestablish communication, a sincere apology letter will go a long way in allowing you to mend fences and bring the relationship back to life.