1) This HAS been asked a LOT before so please try to search. Other than that, I know you're new so I will summarize below.
2) AAMC tests usually have a very high correlation to the actual MCAT score (basically your avg MCAT score on AAMC FLs #7-10 is the most accurate in terms of a prediction).
3) Yes, the MCAT is "harder" than the practice tests. This is not completely true for two reasons. Obviously the real thing seems more difficult than the practice tests because it matters more and nerves seem to play a large role. Furthermore, when you come out of the real thing, you will usually feel you completely failed because you got X, Y, and Z wrong. But this is just because it matters the most and the test is still changing from test to test by emphasizing different topics and varying the difficulty on those topics. This causes a little more problems when combined with the nerves factor.
3) This leads me to the third concern. The way you feel has ABSOLUTELY NO correlation with your score. This has been proven by hundreds of replies on SDN. Most people think they got a 20, but they usually end up getting +-2 of what their average on the AAMC FLs was. Some people think they did well, and they might have, or they don't do as well as they like. Some people think they did badly, and they do badly. Overall, feeling has no determination on your MCAT score not only because you are a bad predictor of your performance, but also because you don't know the curve so your worst section could end up being your best, which happens often.
I hope I summarized everything you needed to know and more.
