First, as werq34ac mentioned, the focus isn't always technical, but in my case, there actually were some fundamental problems with my technique, which I hadn't realized, and having my teacher point them out and fix them made worlds of difference. It sounds like you've been self-taught, so maybe there are some general technical things you're getting wrong. Also, I think there's more to technique than just playing the right notes; there are so many nuanced sounds you can get out of the instrument, and I think my teacher helped me (a) realize that, and (b) really figure out how to control that.There are a huge number of different dimensions along which your playing can vary--the notes, tempo, dynamics, color, on down to things harder to define, like mood, emotion, etc. For any piece, you can improve in all of these aspects and get closer to a sort of ideal version of it (which may or may not be the same as anyone else's ideal version). But, sometimes it's hard to figure out exactly what aspect you need to improve (and how to do that) in order to move closer to your ideal. So instead of thinking "Ok, this needs to go faster," you think "Hm... this could sound better somehow... I think..." For me, this was where my teacher helped me--pointing out specific things that I didn't even know I was doing wrong, and getting me to fix them. Which, on a week-by-week basis, made the pieces I was working on sound so much better, and also made me better in general in the long term because I started to learn to pay attention to all different aspects of my playing whenever I learned new pieces.
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