That's a powerful letter you've written. Good for you. It seems you're at Step One. Now: Are you in AA and/or NA? Do you know what Step One is? I'm going to assume yes to both. How about a step 2 letter? Do you believe in a Power greater than yourself? Do you believe it can restore you to sanity?
You've got a lot of hard work ahead of you. You're going to be getting along without the chemicals with which you greased the wheels of life. It won't be easy, but I can promise you this: it'll be worth it.
As you said in your second post, you're not alone. We all took our last drink and sobered up. I don't know much about drugs, but other members do.
Don't drink or use, no matter what. There is no situation so bad that a drink or drug can't make it worse.
I have over 15 years sobriety.
Remember, I'm pulling for you. We're all in this together.