Your letter made me think of journaling and how theraputic it can be. I hope that you will keep your letter somewhere.....in a journal perhaps or in a folder on your computer? Or here on SR.
I find it very helpful to review some of the "letters" I've written but never sent. It reminds me of how I felt at that time and gives me perspective on how I feel now. It also helps when I find myself falling into those old patterns of behavior that aren't healthy for me (or the addict in my life for that matter;-).
Addicts aren't the only ones who relapse......codependents do too.
gentle hugs to you.....this is hard but I have faith that you'll work through it.