It's probably true that if they feel you're not doing a good job, they'll let you know. It's possible that part of the reason that you feel so insecure and nervous is that the other people around you, being partners, have a much better handle on what they're doing and are therefore more confident than someone who's just starting out. If they say you're doing "good/great" work, take note of why they may have classified it that way, and believe them! If they tell you there are areas where you can improve, work on that. If you stay on top of your game, you should have less reason to feel insecure.
The above is true, but I have to respectfully disagree with the first sentence. Chances are good that partners (or any boss) will *not* let you know when something is wrong. Many will huff off, shut down, or simply steam in their offices while correcting work they asked you to do. You, of course, are left to wonder what's going on. Chances are good that decisions will be made about you before you really have a chance to prove yourself. Fair? Not even. But, as the saying goes, who ever said life was supposed to be fair? In law practice, there's simply too much at stake, and too much going on, for most partners to be sensitive to the agonies of nearly all associates. Fair or not, it's your job to correct yourself. That, after all, is much of what an attorney is: spot the issue, and solve it.
Again, for all, I encourage you to read the three books mentioned: Jagged Rocks of Wisdom; Jagged Rocks of Wisdom--The Memo; and The Young Lawyer's Jungle Book. Not to be overly melodramatic, but they could save your career, or at least months of pavement-pounding as you search for a new job. In this market, these are nearly the same risk.