As much as I'm always talking about accuracy, taking the time to get it right, I want to talk about how important making mistakes is. First off, know that speed and accuracy are mortal enemies. They constantly battle for supremacy in each musician, for both the entire output of a musician's life and for the progress of each individual piece. Lean too far towards getting each note correct and you'll learn 4 pieces a year. Focus on playing as fast as possible and you'll play 200 songs a year, poorly. But many musicians get stuck worrying constantly about being accurate and slow down and thus never develop the speed they are capable of. And that's because you care about getting it right and want to fix your mistakes and that's correct and admirable. But if you find yourself stuck in terms of speed and only learn a few pieces a year, try introducing playing 5% of the time (scales, arpeggios, exercises, pieces) where you allow yourself to make more mistakes. And for all musicians I recommend you take a look at yourself and see which way you lean, Accurate Man or Sloppy Sam.
Concentrate while you practice; imagine you are auditioning for a scholarship or an orchestral position, imagine you are making an album.