Melissa, he did not act professionally, and you do need closure, its easy to say, move on, but its not, you have all those feelings still inside you and thats ok, they are your feelings and you are entitled to them. Closure is a huge thing in my oppinion for healing, whether a person is living or not.

I know you have come a long way, and I congratulate you for that, you are stronger now, you have a good t, and dbt support, that doesnt mean it erases your past thoughts and feelings. I am still looking for closure on stuff that happened many moons ago, I get where you are coming from. Its not easy.
These feelings need to be processed so they dont get in the way of your furthur success. I think another letter will probably antagonize him, or he might take it as a form of harassment, but you should discuss this with your t.
I wonder if your t could intervene, and email him, stating that it is in his and your best interest that he give you an answer , so that you can have closure and continue with your therapy.
In my oppinon to move on and forget the past, is such harsh words( to me) I have always had a problem with that.

I am not saying that to judge anyones oppinion here, because they all give good advice and care for you, that is just one of my pet peeves.
Let me know how it goes.