Have you guys discussed marriage counseling? If you can find a therapist who views both monogamy and polyamory as valid options it might help him work through his dominance/control issues as well as why he seems to react in such a roller coaster way. You two may still end up getting divorced, but it might give you both some peace and closure if you're confident you've tried everything first. And who knows, one or both of you could have some significant realizations about yourselves, each other, and your relationship.
I'm sorry it's played out like this.
I hope whatever happens you're able to do what you feel is best for you.