Hi Micione,
Statistically-speaking, if the ONLY questions that you get wrong on the Exam are the "hard" questions, then you will score higher than a 550. That having been said, it's not clear which questions YOU think are the "hard" ones, so we really need to better assess your GMAT skills before you take the Official GMAT.
From what you describe, it is unclear whether you have taken any practice CAT tests/mocks yet. If you have not, then it would be a good idea to take a FULL-LENGTH practice CAT Test sometime soon; you can take 2 for free at
www.mba.com (and they come with some additional practice materials). That score will give us a good sense of your current strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can discuss how you might best continue your studies.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich