Thank you so much for replying and I'm sorry it took me so long to respond. I read what you'd written and felt a lot better that day, but got swamped by work and what not.
I think what you said makes a lot of sense. The "humming" sort of carried on through the week - my hypnotherapy session did not materialise into what it was meant to be and my connection with my T got messed up (I missed an appointment) because the humming sort of took over everything. But I was working, and that was the priority till yesterday. And the moment I finished, it sort of burst into the real anxieties.
It is, as you said, the change. We aren't ending the relationship, but we have definitely taken several huge steps back - gone back to living separately and being non-monogamous. Which is ... quite unsafe, I'd say, on an emotional level, for me.
Which, as an aside, is really sort of killing me - but even though he said he was okay with staying monogamous if it was so hurtful - only when I said I would walk out - I couldn't... do that, because the meaning of the word monogamous had been so ... stomped in the dirt. *breathes* As you can see I'm still alarmed and saddened and a little bitter.
So this kind of burst on me yesterday and I sat and cried for an hour or so. But it cleared the air. If it is convenient for my flatmate, I will move tomorrow.
I can hope this is a step towards healthy change. But... yes.

I think I'll keep what you've said in mine.