I wouldn't send the letter. She's not going to meet those conditions at this time. Start planning for your life without her (well, not exactly "without her" since you have kids together you will never be entirely removed from her.. she'll be baggage you carry for a loooong time, that's figuratively speaking...), change the accounts now, begin all the other arrangements, and figure out the custody issue. Good luck!
There is no arriving, ever. It is all a continual becoming.
