You can't really create a letter to your past self, telling yourself to correct past mistakes. Even if you could do such a thing, your past self wouldn't even consider to change that past mistake because in that reality, it hasn't happened yet.
If there is one singular true thing that your past self has, its optimism to create mistakes, because your past self at least knows one thing: mistakes are always accountable towards the future, never in the present and never in the past.
A part of you knows this and a part of you tells you not to drink your first shot of tequila, and yet you did. A part of you tells you that you shouldn't get yourself into that relationship, and yet you did. A part of you tells you not to eat that fatty burger just this morning, and yet you did, because your past self knows, that mistakes are always accountable towards the future, never in the past, and never in the present.
So you can't write a letter to your past self to redeem yourself from past mistakes, you can't really tell your past self not to do certain things or to unfuck everything you've done, so what can you do?
Write to the past. The general past, the whole entity of the past itself.
Now writing to the past is not easy. Its like writing to your mistress--not to your wife or your girlfriend, but to your mistress.
Why to your mistress you might ask? Well because like your mistress, you know that you shouldn't be screwing her and yet you do, and just like the past, you know you shouldn't be reminiscing so much of it and yet you do. In fact, you know that your mistress is going to ruin your life and that you should stop having her around before your wife or your children or your family or anyone finds out and yet you keep her because you know, deep inside, that your mistakes are accountable towards the future, its not accountable today, oh god no, not today, because today you're about to have some fun. Your mistakes are not accountable to the past either, because it already happened. Your mistakes are accountable by the future, poor old little future and so here you are screwing her all over again
So yeah, just like your mistress, you like to hide her, just like your past. And most probable than not, your mistress doesn't judge you, hell, that's why she's your mistress after all, she's like that deep sea cave with all the melodious songs of the sea that echoes and surround you. She's that peace and quiet that never comes to tell you what to do, and yes, just like the past--even how much you try to deny it, gives you the same solace like your mistress does, because the past had already happened and it doesn't make sense to have anxiety over something that has already happened.
Or does it?
So write to your past just like you write to your mistress, yearning and wanting for her to come back. Write to the past as if you don't care of your mistakes, you just want your unbecoming. Write to your past because the past is never accountable for your mistakes, the future is. Write to her and sing her a song, so you may be constant with your demise and be lost in purpose with your soul, because your regrets had piled up and there is no way of getting rid of the past and there it will forever haunt you and keep up with you no matter what kind of begging you do.
Now you know your soul has cost and is willing to be sold for the lowest bidder.