Hi, a goodbye letter is something personal and should show you and not some persona, so you should write it in your own language and with a calm tone, so it doesn't trigger the reader.
It usually starts by explaining to the recipient the purpose of the letter, After that, you pass to the reasons of ending your life or tell a story about the theme.Finally you can end them with a quote and a goodbye. Take a look at this for example:
"
Dear [recipient],
I hope this letter finds you in better light as time passes. [Reason for death].
[Story]
Don't let the size of the rearview mirror fool you. The windshield is much larger for a reason. Focus on where you're going, not what's behind you. Don't forget to live, to look back."
Farewell [who u want to adress ex. Dear World],
With love, [Your Name].