Greetings.
Do you only work on pieces and not technique? Because technique practice makes pieces alot easier to play, given that you exercise arpeggios, scales, chords, etc.
Also, how do you practice your pieces? Do you work out them piece by piece problem by problem or just run through them in hope that they will come out? There is a big difference. Your teacher should monitor your practice habits and give you suggestions. Pick out certain problems that you are having trouble in and work on them. Figure out what is causing the problem and find out a way to practice that section. Ask your teacher for guidance. Whether it is a scale passage, or arpeggios, each must be worked in a unique way, slowly, and with full mind control. If your are playing the pieces and still come up with the same problems that means that you haven't worked on the piece specifically or thoroughly enough.
To sum it up: play through a piece, locate the problems, find out what is causing them, and excercise that section. Never rush, and always ask your teacher for guidance, and feel free to ask the forum any questions or concerns that may arise.