Check out this post featuring Rachael Wolff author of Letters From A Better Me

Authenticity
If you are not authentic, don’t be surprised when the apology doesn’t work. If this person is a true friend, they know you. If you are saying something just to appease them, they will not feel validated.
Accountability and responsibility
Be accountable and responsible for your feelings, thoughts, actions, and/or reactions. Do not blame. Don’t say you’re sorry then turn the responsibility of your actions on someone or something else.
If you were mad at someone else, his/her actions can’t make you treat someone else badly. If you were drunk, it’s not alcohol’s fault that you made the choice to drink as much as you did.
The worst thing you can do in an apology is to put the blame for what you did elsewhere. Many people have heard the excuses before, so if you aren’t responsible and accountable, they can’t trust that you won’t repeat the behavior.
Commitment to change behavior
Commit to change your behavior. In order to do this, you need to take a look at the situation and see how making different choices could prevent something like this from happening again.
This shows that you learned from your mistakes and you care about the effect your actions have on your friend.
Re-establish why do you value their friendship
Let your friend know how much you value the friendship and why. When a person feels wronged, they often feel disrespected, amongst other emotional triggers. If they don’t feel like they hold value in your life, they may not find it in their hearts to keep you in theirs.
You want the recipient to know exactly how much they mean to you without using manipulation or guilt tactics, remember to be responsible and accountable.
Once you’ve written the letter, give the person time to process and absorb what you’ve said. If you get concerned about them not responding right away and react by bombarding them with messages, you can actually push the person further away.
Stay committed to making the changes that are necessary, because no matter what happens there is a valuable lesson to be learned.
Check out the rest of the article here: https://upjourney.com/how-to-write-an-apology-letter-to-a-friend
Letters From a better me
How Becoming an Empowered Woman Transforms the World
THE EMPOWERED WOMAN follows the belief system that is best explained in a quote by Byron Katie: “The most attractive thing about the Buddha was that he saved one person: himself. That’s all he needed to save; when he saved himself, he saved the whole world.”
The letters work in different ways depending on the chapter and part of the book.