Make no mistake, what we create as knitters IS an art form. We apply energy and thought and love to nothing more than sticks and string and make something beautiful or useful AND beautiful.
Having been a professional artist for a long time, I can tell you that the two things an artist MUST know are where to start and when to stop!
If you spot a mistake on a finished piece, or one that's way far back from where you're currently knitting, fix it if you can. If it's not worth the effort to ditch most of what you've done and the error isn't all that glaring then go on and FINISH it. Then LET IT GO!
There is another creation just waiting for you to do and next time, with what you've learned on the latest project, you'll do even better.
If you're unhappy with your finished pieces, study up on finishing techniques or just plain pay attention when you're finishing and make sure you haven't neglected or slopped over something. Do that and I promise you're pieces will improve!
Meanwhile, lots of love to all my knitting artist friends out there! What you're doing with your art form is sorely needed today in the world. So keep knitting!