I felt that way about Conlaw when I took it. It was my last exam, and I felt horribly unprepared. I hadn't made my own outline, but instead printed out 2 or 3 outlines, one of which I had spent some time updating, I printed out class notes, tabbed and indexed the lot of it, and that was it. I somehow got an A. I think my analysis of the problems was good, and despite not understanding every nuance of the dormant commerce clause.
So don't freak yourself out too much, or you'll perform worse because before you walk into the exam, you're thinking you'll do horribly. Just know that you're not the only one who has felt like this (it seems REALLY common for Con law, for some reason). Go in, do your best, and then move on to the next exam (or celebrate finishing 1L, whichever comes next!)