She was fine with you being poly when she didn't care about you, but now that she cares for you...

Not OP, or wife, though in a remarkably similar situation. Was a secondary, said oh yes, I'm absolutely fine with you seeing other people. My immediate previous relationship was poly/non-monogamy so I knew it was something I was comfortable in, and even enjoyed some parts of. I exactly said, "No worries, I'll just sit with you until you find Her."

Twelve years pass, and he does not explore any part of it. I begin to think I might be Her. Now he says he wants to explore relationships with other people. I come unhinged. He is shocked, saying this has always been the understanding. Why am I upset? To this day, he honestly has no idea.

I come unhinged not because I don't care about his needs. I care very much about them, and by all evidence, I'd been meeting them for X period of time. In a laughable under-statement, I have become attached.

Replace the word "care", with "attached" and that is me. I exactly said, "I wasn't attached to you before, but I am now." Being told, "now I have more needs and I do not want you to meet them. " was a bucket of ice water shocking.

I do not want this relationship to end. I've been caught completely off-guard because I thought the relationship was going really well. We have made lifelong plans. Out of the blue, he says he wants to see other people? I was not aware I was still considered temporary. So I reacted emotionally, very, very emotionally.

Back to the OP, I am not your wife, and I can only offer my perspective. My relationship is a terminal illness. I waffled back and forth on whether to leave now, and get it over with, or enjoy the time I have and see if I survive. I have decided to see if I survive. He could go out looking and realize I am the person he wants to spend the rest of his life with. I think I'm pretty cool so I have hope that I will. But this decision cost me a lot. I have been going through a detachment. I cannot be as invested in him as I was before. He's happy because he can explore his other options now. I am not happy, but it is the cost of being in a relationship with him. In his mind, that has always been the cost. I should have known. After twelves years, mortgages, combined finances, well, I hope others can see I might have made some assumptions.

I have made the choice to stay, knowing that every new partner could be Her, and my relationship with him will end. I have to be open to meeting other people because he is actively looking. I'm still not ready to do this as it will definitely mean the end of our relationship. I am monogamous. Opening my heart to another person means closing it off to my current. I have no interest in that now, nor did I ever plan to. But it is what needs to be done to prepare myself.

There is no compromise, there is only her going through the grieving that her attachment to you, does not match your attachment to her. You can put boundaries on it, make deals, give and take, none of that matters. You want to see other people, possibly move on from her, and she does not want to move on from you.

I was going to delete this, but I think I'll leave it as is. It is the brutal truth of being monogamous in a poly relationship.