Hi Molly
I'm sorry youve been so let down, and I understand our need to let him know how you feel. The thing is he does know and he doesnt want to deal with it because then he would have to recognise what his selfishness has done to you. Until he is able to recongnise this he wont care what he has done, he is in the throws of utter selfishness and is calling all the shots.
You writing that letter doesnt tell him anything he doesnt already know, and in a way gives him sway and power over you, it's like saying oi here's a great big scabby wound you created come and pick it when you want, if you must write it then either do so and then burn or destroy it, or better still write it on here, that can be quite cathartic either way.
The ultimate power trip is for you to pack his shit stuff and tell him now in your time and not in his, your making it easy for him by being able to plan his exit, give him the exit you want, whats best for you and the kids.
He's heading for a car crash relationship, step back and watch it unfold I give it 3 months tops for when the shiny bits start to fade and reality sets in, meanwhile you'll be 3 months down the road of thank fuck he's gone new hair do and peace and quiet.