Let's start with the one that is, in general, most powerful...

When most people perform well they attribute it to skill on their part. (Maybe they are too inclined to attribute it to personal skill, but that's a topic for a different, much more cynical post.)

But if you're dealing with impostor syndrome, this natural tendency to assume you're a virtuoso is on the fritz. You do a great job and the default attribution bucket isn't skill - it's luck, overwork or invisible elves that accomplished everything while you were napping.

Many interpret enactive mastery experience as "keep working hard and you'll see it's your natural ability that's causing the results." If that was true, impostor syndrome wouldn't exist. In fact, if you don't actively change your default attributions, merely seeing yourself succeed isn't going to fix impostor syndrome - it's going to make it worse.