Urrrggghhhh OP
I feel your pain!
So he passionately understands (which is I think what you meant?) that you deserve to be treated with respect. Yet he still seems incapable of actually doing it.
Never mind -- setting aside the questions of what he means and doesn't mean, what he is or should be or could be, what his real intentions might be, etc... Just considering this is interesting:
He wants you to give him another chance, over an unspecified time, starting with total capitulation to his needs. That already weighs like lead, because it's him. And he desperately, passionately wants it. So that now starts weighing, ooh, more than uranium. No wonder you feel crushed by expectations!
You don't love him any more and would rather be with someone else. But that weighs like feathers, because it's just you. (Also you have no right to feel it because you committed a Very Bad Sin.*)
Add in his glib logic, and you don't have a leg to stand on.
It would be fun to consider what things would look like if lead, feathers and uranium were swapped round. If your side was given all the weight, and his was just feathers. You'd be waltzing out the back door right now, hardly pausing to say goodbye.
Of course, in a healthy partnership each party's feelings and desires are given equal weight. That's an interesting scenario to try and picture, too. Derek probably lacks the emotional maturity required to find a properly civilised way forward, but it's encouraging that he promises to play fair in the event of a split. (I hope he means it, but you are the best judge of that so I take your word for it.)
I do hope @peridito is right that an amicable separation is possible.
Have fun with your friend, and please tell your mum! 
*Just to clarify, it there is such a thing as sin, I don't think it's having an affair when your husband has made you utterly wretched for years. The real moral crime would be to ignore the desperate cries of your soul and fixate on fake love instead of trusting your instinct for the real thing.