You must never take responsibility for anything he has done. Your newer posts hint that there have been unpleasant incidents in your past together, which both of you have excused on his MH problems. You say you've allowed things to happen, but I don't believe that. From how you've described your conversation over this horrible latest incident, it sounds like you two have a pattern going where you try to assertively initiate an adult conversation when he's been unacceptable - which, incidentally, is a very healthy and mature attitude and one counsellors would approve of, so hardly creatingproblems yourself - and he convinces you that you're imagining things, making mountains out of molehills, misinterpreting, causing a fuss, being oversensitive, etc., and then, if you 'dare' to continue, he admits the fault but says you caused it and then to get past it because you are at a stalemate, you both decide to blame it on his MH.
The thing where he said he didn't know how far his boundaries could go when angry is chilling. Keep that one thing in your mind. Out of all his lying, vagueness, shifting the blame, etc., THIS is the one absolute truth he has told you. There is a saying; people show you who they are. All the other crap was him not wanting to have to accept responsibility and saying whatever to shut you up but that was him showing you who he is. Raping you is showing you who he is. Being awful and then making it someone or something else's fault is who he is. Yes, he can also be a man who can look after his children well and show them love and someone who can be nice to you too, but the core of him is rotten and dangerous. Nobody is all good and all bad; just that some people have too much bad to justify accepting the good, and he is one of them. As for is MH problems, even if he suffered from a condition called Rape-itis it still wouldn't mean you had to tolerate it and stay with him.
Look at what he has done; he has actually admitted to raping you and is totally minimising it and somehow blaming you. You're finding it hard to get your head round it because their is no getting your head round it. A normal, decent bloke wouldn't even dream of doing such a thing to his partner, much less try to make some sort of excuse for it whatsoever. The bit about him saying he couldn't see a need for it to happen again is also terrifying to read; that's basically saying "If you tow the line, I won't need to punish you by violating you again. Be a good little wifey".
You have done amazingly and come on so far in such a short period of time, it is so impressive and positive to read aout your new developments. Offred is spot on, however, when she says that you should concentrate on the facts that he has abused you and refused to take responsibility and go on from there. Trying to work out why he's been how he has been is fruitless and does not help and does not solve anything. All it does it keep you there with him whilst you drive yourself mad trying to work it out and the longer you stay, the more normal everything starts to become again and you risked being sucked back in and making excuses for him and letting yourself be blamed and violated. You won't make sense of it because you are a decent, normal person and his actions can't ever make sense to decent, normal people. In time, counselling would help with this and with helping you break down patterns you've developed for accepting blame for other's bad behaviour. I think you've been fantastic and keep up the good work and keep us updated. We really care about you.