I've discovered recently that I really struggle when relationships move "too fast", and not just my own. It is associated with my dislike of change. Even the slightest change is magnified. I even find positive change hard.
My current relationship moved fast. Too fast. And I just about handled it without freaking out. He disagrees that it moved fast so no surprise that he's met someone new and it was/is moving fast. I am/was struggling. I know now that this is a burden of being with me in a poly relationship.
I've said that if it's too much, I'll step away and let them stabilize then perhaps come back.