There are many ways to slow down:
Apply the brakes
Take a deep breath
See the big picture
Go for a walk
Jump off the moving ______
Regardless, of which way you do decide to decelerate, you should know why you are taking that action.
Is everyone else doing it?
Is it time to move on?
Is it no longer worth it to put forth the effort?
Whenever I have thoughts like this, I double down and go faster.
If I'm still having doubt, then I'll consider changing the way I work. Very often, pushing through the doubt opens up a new world of possibilities because I've shattered my perceived limitations--and likely the expectations of others.
Push even further and you may find yourself up front and leading the pack. That's not a bad place to be.