What you say strikes close to home - your description (and the description of addiction) sounds like what passed, in some ways still passes, between my XW and me. It left me continually asking "Why" and unable to move on because of that feeling that we weren't done.
It's the strong emotional connection and it can be a killer. If he won't end it, you'll have to because you can't continue an emotional connection with someone who won't/can't be with you.
Like Ali noted, he probably doesn't love her but loves you, just like my XW doesn't love her "muse" but probably still loves me. The problem is, neither of them can be there for us. There is something in them that prevents them from being there for someone else in a long-term relationship. And so they sabotage. Have short-term relationships. Never get too close. Focus on themselves. But deep down they probably know how messed up they are - my XW kept saying "What's wrong with me?! If I can't make this R work, I won't be able to make any R work."
But we can't do everything and if they can't be there for us, then we have to accept that fact, cut the ties, and move on. It isn't a "sick" situation, necessarily, but just one of incompatibility. All R take work, but when the R is capable of being long-term, both people are willing to do the work, not one always running away and the other being left to prop up everything.
