The exhaustion and absent-mindedness will eventually pass, plucking. I spent a few months like that last year. I was depressed and on ADs at the time, and trying to make sense of my life having come crashing down around me (miscarriage, realizing that then-H was violent and abusive, leaving him, then realizing that my parents had trained me for abuse).
I forget: are you getting counseling? If you are, your counsellor would be an excellent person to talk through your letter with.
Mail the letter when you want to. It took me close to a year of toying with the idea before one day I just sat down and wrote it in one jot, and sent it. I don't know why it was then, but that was just the moment I was ready. Perhaps, as your husband astutely says, because that was the moment when I no longer cared what they or anyone else would think of me writing such a letter.
The reaction I feared the most, in fact, was not my parents' , but the one from 3 other family members who I love dearly. But I was ready for them to cut me off in turn, if that was what they chose to do. Their reaction turned out to be beyond my wildest hopes: they only heard my parents' side of the story, and even then they understood and called to tell me that I had been very courageous; that they wished they had done something similar themselves years ago; that it was a very loving act, in fact, to give my parents a chance to take a look at their actions and acknowledge my feelings. But then, they are people who have taken a hard look at abuse in their own lives, so they understand the need to break out of the system.
Last night I dreamt that I was shouting and swearing at my mother, and it felt good. It's the second time I've had a dream like that, and they are very vivid. Except that in my dreams, my mother shuts up and listens. Which is not how it would go in real life.