After my break up with my ex--which was really abrupt and weird--we went NC. During the next few months he wrote me a couple of short emails telling me he loved me, to which I never responded.( He actually broke up with me so I felt like he was either drunk when he wrote them, or just effing with me, probably both.)
My AV started acting up a few months after that, and I used the fact I hadn't responded to his emails as an excuse to write to him. I showed the email I wrote to my friend who went through AA for a couple of years (turns out she wasn't an alcoholic-- long story) and she told me it was the healthiest I had ever sounded, but that I should NOT send it. I did it anyway.
In the letter, I told him I was as sick as he was in that I had become very addicted to trying to control his disease, and that it was wrong of me to tell his brother he had a drinking problem. I told him I was trying to work on my own recovery and to accept the toll my unhealthy relationships had taken on my life. I asked him to not send anymore of those lovey emails and wished him well.
The email I got back from him totally shocked me. Not a single thing I said got through to him. Six months of NC and he was not any closer to honest communication. If anything, he seemed even further entrenched in his denial. What shocked me the most though was ME and the fact that after six months, I STILL thought I could penetrate his disease. Letting go of trying to get through to him was (and still is) my biggest challenge.
Anyway, a week long exchange of frustrating and depressing emails followed, and I was upset and unsettled for a few weeks after that. Writing him was a mistake, kind of like a relapse for an A.
I can't figure out why your therapist thinks you should actually send the letter. I can see the benefit in writing it, but sending it is so different. Does she have experience with alcoholics? I would ask her what she thinks you would gain from sending the letter. And I suggest thinking about it. The decision doesn't have to be made right now.
HUGS