I don't know if I am breaking a rule replying to your post, as I am a woman, but I have done this. My husband is a SA survivor, and I have written a letter, (not to be sent anytime soon if ever), to his abuser. (He is still working through his emotions, but planning a letter of his own.)
It took me a few weeks to finish my letter to my satisfaction. I just started with key things I wanted to say and thoughts or emotions that I wanted to make known to him. I revised it when I thought I had everything in there and saved it. That was one of the most wonderful release! I just vented, ranted, raved, cried, and mourned.
I think the most important thing is to be honest. (You can write something as to how strong you feel later.) Don't fill it with B.S. or that is all you will see it as when you read it later. When you get overwelmed with your emotions and flashbacks, take a break. Don't push yourself. If you want to wait on this letter until you feel stronger, do so. Until then maybe keep a "feeling journal"? You know, how you feel that day (or week), what problems you are facing, how you're dealing with them, and any accomplishments you have made on your journey. As for actually sending them the letter, that is truly your decision. But by sending (or giving) it to them, you run the risk of a confrontation. (Those usually don't end well.) You might start a poll. "To Send or Not To Send, That Tis The Question"
Keep your head up, things do get better. You have already shown strength in your desire to write this letter. I admire that. Keep us posted.