We must be careful in our study to be interested in what helps us and not what hurts us. Often, when we practice we become focused on what is negative in our playing. There's nothing wrong with noticing what needs improvement, but we want to make sure that we are interested in what makes us better - and not focus on the problem. By this, I don't mean to just ignore mistakes, but to make sure we are continually focusing on what we want.
For example, let's say we have two students practicing a fast scale at 120bpm and both of them are having problems with part of it. Student #1 may think "I can't play this scale this fast - this is hard!" while Student #2 may think "I made a mistake, but that's ok - I want to play this scale fluidly"
In both mental attitudes, the students may slow down and work on the scale, but with Student #1, the interest is now on the difficulty, while Student #2's interest is on playing the scale fast and fluid.
This might seem trivial to some, but understand the fact that "The only thing that can grow is the thing you give energy to" (Emerson). Both students may physically work on the scale in the same way, but because one is looking at the difficulty while the other looks for the solution, the results will be different. Student #1 may keep thinking about how hard it is and tense up every time the scale is played, thereby making it harder to play and reinforcing his belief that "this is hard!". Student #2's search for fluidity gets him to eventually relax and enjoy the feeling of speed. As he keeps relaxing, he builds the belief that "this is easy".
I know this example is black and white. Often, we have a mix of emotions while we practice. For example, there are probably days where Student #2 feels that the scale is too difficult and doubts his abilities. But if he can keep the majority of his thoughts on what he wants, and continually imagines what he wants, and keeps interested in only what he wants, with time, what he wants will start to appear.