I'm coming in a little late to this post. Don't know if this is helpful..
I've written a million letters to my addict. It didn't matter either way if I sent them, but the process of writing it helped me.
In the case of my addict, the need to write a letter now tells me that she's not listening to my words and I realize a letter won't help either. So I normally don't bother and instead work on why I'm still holding on.
So, if writing helps me let go or gain peace, I do it; if sending it helps me, I do it. But I try not to have any expectations that it will change her in any way. Nothing has worked in 4.5 years.
Sending or not, I hope you find peace and detachment...