Glad to see you back around these parts, but not for these reasons. I'm so sorry things are this crappy now.
I have no advice. Wish I did. Like you, I was the "good" wife in a lot of ways. Did all the laundry, did well over half of the cooking, provided most of the child care... and to boot, I brought home a substantial portion of the income, and sex was never much of a problem with my H until he got into this wild MLC/addiciton problem of his. And then I got a kick in the a$$.
So, I don't know if you should give him that letter or not. But... sometimes we need to get real with our spouses and give it to them straight. I hope things start looking up for you. Take care of yourself.
