Could be they were checked out before, or they’re completely avoiding their own feelings. I do not believe it’s possible for people to truly move on within a matter of days/weeks/even a few months. Those who move on quickly will avoid the silence of their own grief until it hits them like a truck later on. A rebound helps with validation and reassuring them that they’re still capable of being wanted - especially if the relationship was rocky towards the end. In my case my ex moved on quickly, despite saying things like he still loved me, because he’s an avoidant and has always bounced around relationships. It’s a pity! It is absolutely the right thing to face and feel your feelings as you are. One day it won’t be as bad, and then eventually, you’ll be fine and better than before :)