I agree Craig. I think letters are often a bad idea, especially explanation ones. It's all too likely that she'll take it in a negative way, plus she already probably knows why you felt this way. Plus if she doesn't respond, you'll keep wondering what she thought, and may turn it into a pursuit.
My C told me to write all the letters I want, but don't send them. I have a drawer with several. It helped to articulate my thoughts and feelings, much like journaling here, but my W doesn't want to hear this stuff right now so I leave them be.
Me: 43 W: 41 Together 2009, Married 2011 Sons 10 and 6, Daughter 5 Bomb 2/21/21. W moved out 10/2021