If your partner makes a dramatic shift from being upset with you to suddenly being “over it,” they’ve simply ignored and avoided their upset feelings. This is the worst thing that can happen for you personally because it’s a guarantee that their bitterness and anger will come back to you at some point, and probably sooner rather than later. Your partner can’t shove their feelings down indefinitely; those feelings will need to be dealt with at a later point in time. If they aren’t consciously dealt with now, they will come out in unconscious, unhelpful ways later.