I’ll give you a sure way to mess up your practice and get discourage: take it from the top!
So, you have this song, this exercise, this page to play. And, as is logical, you put your hands on your instrument and you start to play it. You know, you do that just to get the feel for it. It’s very natural.
And what’s wrong with that? you might ask.
Nothing, really, if the goal is to get good at playing the beginning of the piece.
Because, let’s suppose that you start your piece, as indicated earlier on. But then, you make a mistake. What do you do? That’s right! You start over! And so, here we go again, from the top! we are playing, we are going fantastically well, until we come to the same passage as before. And, Bam! Crash! Kabloey! we have to stop, because the mess up is so big, it could be heard from space. Maybe, in between the two version, we made a 16th of an inch of progress. Not more. 1/16th of an inch. And that’s a maybe!
That little progress is what’s teasing us into keeping applying the same method and starting the piece from the top again and again. We are making progress, this is working, THUS! we shall plough along the same course. From the top.