I can only sympathise. I am expecting a baby with an N boyfriend. I truly thought he was everything I ever wanted and the fact that he isn't at all is hard. When he is being nice (ie normal) I want it to be okay so badly that I get close to giving in and going back to him just for the nice bits. I even talk myself in to putting up with his crap just to get the affection and cuddles (especially if I am doing things his way). I too have tried working through it amicably and tried to be reasonable and like you have days where I am met with perfectly normal behaviour. How are you? How's the pregnancy, if you want me to help in any way, just say , .... etc etc. Closely followed by days where I am told that "you are the type of person who needs someone", "you have hurt me more than anyone ever has" I don't deserve this" "you wouldn't have anything if it wasn't for me" " I would not have gotten you pregnant if I'd known what you were really like" and worse. I know that my family will be better off without him, but like you I try and make it work.
I think what we are doing is giving an n person, normal traits. What I mean is, we are treating them as a normal person and expecting them to act that way. I just find it so hard to accept that he is like that. His nice bits are the best, but his n bits are so awful I know I can't cope with them. I have broken up with him almost every day for two months now and I am so drained by it all. I don't mean that I go back on my word, but that he doesn't accept it and ignores what I say and then we have the same thing all over again the next day. That's why I think that if you go for the legal route, at least you don't have to worry about his N traits affecting your day to day life. You can stick by what is legally binding and if he is having an "off" day, you don't have to worry about it. I am thinking along those lines myself.
I hope you sort it all out, and if you want to let off steam and just get something off your chest then feel free!