Ugh. I am so sorry!!!
The only thing I have to offer you is how I have moved through the process when coming to a similar crossroads in my life. I realize that having those second thoughts is simply because I don't WANT to go through the hassle of making those necessary changes. Change is hard and painful at times. But the end goal is what I have to keep in mind...and in order to get there I have to get through a lonely dark tunnel. I accept that the tunnel is going to be difficult but temporary and then start on my journey one step at a time. One day at a time.
Without knowing you or your husband, my feeling is that when he sees you are serious and in fact obtaining the necessary boxes and luggage, etc., and beginning to pack, he will hopefully realize that the children's needs should come before his own and he will leave. Your words need to be backed up with action -- it's critical at this time--just as you expect his words to be backed up with action.
On the other hand, you could "wait and see" if his letter means anything at all. You can have another round on the ferris wheel if you want, but experience probably tells you that you will be in this very same spot a month from now. And then what? Might as well get started through that tunnel. It's a necessary part of the journey and it begins with acceptance.