I agree wholeheartedly with K D however, I did write a letter to my ex several years after he left. Years that I had felt as you - angry, hurt, bitter. I could not imagine forgiving him as he not only showed no remorse but also continued to inflict pain on us. It only kept me stuck in one place - and not a good place.
One day I saw a sign up outside my church
"HE WHO ANGERS YOU, CONTROLS YOU"
That hit home with me. He never even knew how angry I was day after day.
After another of our bouts of him not paying for D's education, dance and costumes, grad stuff etc - I finally wrote him a letter.
I talked about how I was letting it go. That although I could not agree with what he had done or the way he was treating us - I was stepping out of the drama. And he could carry on till the cows came home but I was not going to play anymore. Ashley was his daughter - but if he chose not to help pay for her - that was his choice. The courts would sort it out in time. But she would not go without. Because I would take care of her in the best way I knew. She would not lose out because of the choices he made.
I told him I did not want to converse with him anymore. I had sole custody - he had some visitation. There was nothing to talk about anymore.
And the drama ended.
It was a long time later that I showed up at Emergency with my son and was told he was not likely to live the rest of the day. Then and only then did I contact him and at that point - I only cared about doing the right thing - one parent to another. His presence was very minor to me as Ryan was the only one on my mind.
From then on - we have communicated by text regarding Ryan. (as I'm sure you know - he has not only survived, but thrived since that horrible day).
My point is - if a letter will make a difference - do it. If you are just telling her that you forgive her and are letting go. I would save my breath. Because actions DO speak louder than words. And time heals all wounds. Never thought it could. But it does.