Short answer: after a serious breach of trust, an apology letter should name the harm plainly, avoid asking for immediate forgiveness, and never use remorse as pressure.
Accountability is specific
“I am sorry for everything” can sound like an attempt to close the subject. A more responsible apology identifies the behavior, recognizes its impact, and avoids explaining it away. It does not require the hurt person to comfort the writer.
Do not promise what you cannot demonstrate
Promises such as “I will never hurt you again” may sound dramatic but offer little. If you describe change, keep it concrete and truthful: a boundary you have set, a conversation you have had, or a habit you are actively addressing.
Respect the response you receive—or do not receive
An apology is complete when it has been offered sincerely. Forgiveness, reconciliation, and continued contact are separate decisions. Do not follow a letter with repeated messages asking whether it worked.
For a structure example, see this accountability sample.