I feel like it's pretty much universal that you're going to think you did worse than you actually did. I feel this is mainly due to the fact that you don't get the instant feedback of your scores like you do during FLs. You're going to dwell on the difficult parts of the test, and blow things out of proportion. The real deal is also likely to be a touch harder than the practice tests. Unless you didn't get to a passage, or were guessing wildly on a significant number of questions, I think it's fairly difficult to be able to determine your level of achievement in the heat of the moment.