The essential elements for effective practice and musical growth are consistency, objectivity, and discipline.
Consistency is simply the goal of making small daily improvements. Musicians and athletes can't "cram." The mind and body require time and consistent training to develop a world-class level. If your goal is to win a major audition, or become first chair in the all-state band, then begin working on your goal a year in advance. Muhammad Ali once said, "The fight is won or lost far away from witnesses, in the gym and out on the road, long before I dance under those lights." If you make small daily improvements, you can become a powerful force.
Objectivity means approaching your practice uninfluenced by emotion or personal prejudice. This doesn't mean to perform without emotion. A passionate, emotional performance is our goal and will move an audience. We can only get there through objective practice, which is really hearing what you are playing. Recording some practice sessions will help with this. If something doesn't sound right, focus on what needs to be done to make it better, rather than letting it upset you. Do this by imagining that you are advising someone else. When you let go of negative emotions, you will also let go of excessive tension and find that you can stay in the moment. As you achieve this, you will be getting out of your own way.
Consistency is most effective when it is combined with discipline. Disciplined practice means working on things that you find difficult and setting a high level of acceptable proficiency for the material you are studying. This doesn't mean that you never play anything fun. Always include a little "dessert." Your main focus at any time should be on the hard stuff. If you keep your objectivity this will be easier to do and your skills and confidence will grow rapidly.
A musician's level of performance will only rise to their level of expectation, so begin to expect a lot. Don't accept something because it's "good enough." Work over time to get closer to that ideal level of performance and you will begin to achieve it.