My best advice is to make a list of all the reasons why you want out and the situations that brought you to this decision. Keep it with you. If you are truly done, don't engage in conversation with him about the situation. Keep it business, who is doing what and where to go from here.
I am in this process with my STBXH right now. When he starts talking about how we got here and how to fix it, I just shut him down and tell him I made my decision.