Welcome to the morass of professional life.
Is your career progressing? Have you accomplished anything in the last year that you'll put on your resume? Yes? Good, continue. No? Consider other options.
Is the project proceeding? How does your manager feel about your work? Good? Maybe it's a perception problem on your part. Anxious? Perhaps you're not being applied well and should be moved to a different project within or without the company.
Things checked out OK so far, but you're still not happy? Perhaps you should consider trying to flip your role in the company to something that would be more beneficial to both you and the company.
My productivity plummets during certain phases of projects. If you're feeling that way perhaps you just need to get past a milestone onto a better or more interesting task.
Don't panic about it! Some corporate structures can withstand an almost malicious amount of unproductive behavior; in fact, managing the spectrum of productivity is in large part what corporate management is about. That said, don't abuse it -- long term it will definitely hurt you more than them -- but think of it as productivity insurance while you figure out how you can succeed.