Does this seem familiar to you? A yoga teacher can suggest that you clear the mind and can provide the cues to help you get there, but you are the only one who knows what’s really going on in your mind.
As you become more experienced in a yoga asana practice it becomes more important to bring the meditative aspect into your practice. As you become more adept at focusing on the pose at hand you become more adept at focusing on whatever task is in front of you off the mat and excluding extraneous thoughts. That allows you to give more of your attention and best efforts to whatever you are doing.
Each time the mind wanders off track in your practice gently bring it back to the task at hand, and to the breath. Feel your body fully. Feel where you feel tight, where you feel good, feel the quality of the breath as you hold a pose. Notice the differences in the feel of each pose as you move. Notice if the breath is moving freely or is feeling stuck at any time. Relax your mind and be with the pose. Over time the as you let go of thoughts, thoughts start to let you go.