To build on what Sopher said, it's important to separate practice from playing. When playing, if you make a mistake, it's important to play through and minimize the impact of the mistake. When practicing, you want to focus on the mistake, figure out what's causing it. and practice that phrase or section until you can eliminate the mistake. For me, it's the hardest part of practice. I'll often find myself rushing through a difficult section to "get it over with," when what I really need to be doing is slowing that section down and figuring out why it's giving me trouble. Sometimes it's poor technique, sometimes it's just practicing until it feels comfortable, and sometimes it's just bad choices (i.e., playing a higher note on the same string when playing it on an adjacent string would make it easier or cleaner).
To PereBourik, I know you're taking lessons from a top-notch teacher, so be brave and bring your sloppiest stuff to her and get advice. We often want to impress our teachers with our best stuff, when we get the most value bringing our biggest challenges to them.
I had been playing banjo for about a year when I started taking lessons from Tony Trischka. I wanted to impress him, so I learned his version of Train 45 and played it for him. My heart sank into my shoes when he quietly shook his head and said, "Sounds like we have some work to do!" We spent 30 minutes focusing on the first four bars of the tune, with Tony showing me a much more efficient right-hand approach. I learned more about playing the banjo in those 30 minutes than I had in the year previous!