How to reconcile plans with results?

Have you ever been in a position where in an interview before a job, you were so confident about all the positive changes and impact you'd bring into a company when you join. Two months in, it's not going as great as you pictured it. How do you reconcile that with your skill level? Or do you even reconcile it at all? For me, I get pretty down when I am unable to do my job as good, especially with things I said I'd do. Do you ever get like that?