So kudos for being honest so far. Given all that's been going on, I think you deserve some credit for that. For not cheating, and for the multiple efforts at trying to fix the situation.
And I agree with everyone else that you need to stay that way. Don't cheat, it'll just make everything harder and create new barriers. Or make you beat yourself up, which thus far you don't seem to have cause to do.
Sounds like you've really tried communicating with your husband; not clear whether he really doesn't get how important all these issues are to you or whether he just can't do it. Maybe counseling could help you get through to him in a way that you haven't been able to on your own, but I admit that my gut reaction is that you just have very different needs/ideals, and he may not be up to meeting yours. Do you have a good idea of how he envisions relationships? Is he really happy with how things are now? If so, to make this work, someone is going to have to go against the fundamentals of his/her nature, and I'm not at all sure that's a good idea.
FWIW I totally sympathize with wondering whether you are truly polyamorous or just feel that way in response to the situation. It's hard to dissociate yourself from the situation.
So do try communicating, really opening up about all of this, or at least about how much of a problem it is for you. You can't keep going like this, even if you love your husband; it's just not good for you. And maybe just focus on that communicating, more than on deciding what the ideal relationship structure is for you. But I don't think you need to jump immediately to divorce; even if you end up there, you'll feel better I think if you can look back at having tried all you could.