In response to Steve McQueen's concern that it might not leave any wiggle room once sent, I've softened it ever so slightly:
W-
No need to get together to talk. Your silence speaks loud enough. Your behavior over the past year has been unbelievably disrespectful to me, our marriage, and our kids, and now you making me wait while you "decide" what to do is the final straw. What is a few days possibly going to change? I made myself clear. I will not live in an open marriage, so if you choose to continue your behavior our marriage is over.
I'll make it easy on you. You don't need to decide. I've decided I'm done and I'm moving on. I want to finalize our legal separation as soon as possible. I'll scrape up the money to pay for my half. If you claim you can't afford your half of the cost, I'll cover yours too, and reduce your monthly support by $100 until it's paid off. Although I haven't made any definite plan, I might file for divorce in order to get this done sooner. I want to be totally free to pursue a new life, and being married to you for another year under these conditions is a sacrifice I'm not sure I want to make. When someone asks what my situation is, I want to be able to honestly say "I'm divorced".
This is not what I ever wanted, and my heart is very heavy thinking about what the kids' lives will be from now on, but I won't tolerate your disrespect any longer. At least during the time the kids are with me, I'll be able to honestly demonstrate what is and isn't acceptable behavior from someone in their life.
H
That phrase is made bold here for clarity, it won't be bold in the actual message. Just matter of fact.