Makes perfect sense to me. It’s part of that whole wanting to be special and important and needed and loved thing. (I have a theory that this is the source of a lot of conflict between couples; when we’re not getting something we want or need from our partners, the stuff isn’t the real issue. The real issue is wanting to be special enough, and important enough, and loved enough that our partner wants to do these things for us.) You want to be special enough and important enough to her, and loved enough by her, that she wants to have a white picket fence and 2.5 kids with you. And the idea that you might not be is tearing you apart. That’s got to be absolute hell. I wish there was something someone could say to make it feel better.
If being with her on an equal footing with nevermore is going to tear you to pieces, if you’re going to die a little every time they go into her bedroom and shut the door, if the sound of your children calling nevermore “mommy” is going to be a whiplash across your heart, you do not need to be with this woman. If your emotional needs aren’t being met, your soul will bleed to death from a million tiny cuts, and your love for her will shrivel up and turn sour. You both deserve a lot better than that.
I have to wonder about something, though. The linked thread said the threesome ended a month or two ago, but you apparently insisted it end again a few days ago. If we knew why the three of you decided to try again, and why you decided it had to end right after you got a rather disappointing answer to your proposal, we might be able to help you sort through your feelings a little better.