Hi didiervanoverveld,
Many GMATers spend 3 months (or more) of consistent study time before they hit their 'peak' scores - and many of those people will spend 15-20 hours each week on their studies. Since you've been studying for just a month - and only 9 hours a week - you shouldn't expect to have mastered any GMAT concepts just yet. Thankfully, you don't actually have to 'master' anything to hit your particular Score Goal. When dealing with questions that you think are too challenging, you should immediate 'dump' those questions (just take a guess and move on) - that way, you'll have more time to spend on all of the 'gettable' questions. By extension, you should be able to take more notes, double-check your work, etc. and minimize the little mistakes.
You can pick up all of the additional points that you're looking for in either the Quant section or the Verbal section, so you might find it beneficial to focus on whichever section you find easier to deal with (and hone the proper skills for that section).
1) Do you find the Quant or Verbal to be easier?
2) Going forward, how many total hours do you think you can commit to your studies before Test Day?
GMAT assassins aren't born, they're made,
Rich