God knows I've needed time to adjust to things, but this feels like the *same* thing, just different forms. Sometimes, even I just need to rip off the bandaid so I can move forward and not stay in the same holding pattern.
It's great that you said that, but how will you back it up? It is likely she will try to get away with some form of control to test you and see how serious you are when you say that. So, what will be the consequences if she falls back into controlling behavior? You need to make that clear for both yourself and her.
But she doesn't have ownership of it, it's your bed. What's there to "get used to?" This is a bit of coddling you're doing, IMO. I think she needs the Band-Aid ripped off. Time to pull her head out of the sand and shake off her fantasy illusion of monogamy with you, and face reality!
I can tell you that it will always be "just one more thing." I have experienced it first hand. I was part of a budding triad. It seemed fantastic initially. But for reasons no one could ever ascertain, the wife suddenly became insecure and jealous. In an effort to give her time, over the course of several months, she "one more thing'ed" us to the point that her husband and I could not have even a semblance of a romantic relationship. NOTHING ever quelled her insecurities.
Finally at one point, the husband said to her that she would eventually disallow us from even having a friendship. She looked incredulous. Why would she ever do that? In just a few months, she did, though.