People feel they need to hustle because they're getting away with something. They feel that when they get found out, the spigot will shut off.
Imposter syndrome is closely connected with this idea. If you don't have confidence in your abilities, you're going to have the feeling that the good times won't last. If you constantly feel that you're going to be unmasked, you will fear the future.
If you realize that you have value and can provide value, you won't have that fear.
Imposter syndrome doesn't just stop you from starting, it stops you from relaxing.
The opposite is a growth mindset.