dVdT...I think you and my wife should talk.....she's going thru many of the same emotions as you are, atleast that's what it sounds like. We are in a tough place right now, married 27 years but only recently poly with her best friend of 20+ years wo is recently divorced and now lives with us. They are not together, it is a "V" relationship for both of them with me.
We are 3 very good friends. I can't tell if it's the sex or the love, that tears her apart. We've tried talking a lot but nothing ever seems to get resolved. It's very frustrating. I have reassured her in many conversations that I still love her and would never leave her. We didn't make it 27 years by accident. It's just that I think I've finally acknowledged my poly roots and don't know if I can put that Jeannie back in the bottle.
She has recently joined this forum but not posted anyting yet. I will try to direct her this way. Sorry I don't have any solid answers for you, but I'm sure others here will have some ideas. Just wanted to let you know that you're not the only one going thru these emotions.