Be a musical troubleshooter. When some aspect of your playing is not going well, try to isolate the problem. Then, think back to your lessons to some of the statements your teacher made and try to use some of the suggestions s/he may have given to you. Watch yourself in front of the mirror to see if there might be some physical problem. Record your playing with a quality hand-held recorder such as a Zoom H2n or your smartphone. Remember that it is unlikely that your phone can give you the quality of tone that you really produce. Most suggestions made in lessons have been made before.