When I started doing yoga, I always looked at the other people in class and thought they had such amazing concentration. They always looked ready to start class and their concentration was laser focused for the duration of what we were doing.
Myself, I always felt like my mind was wandering, thinking about food, sleep, if other people were watching me or having some random thought. As my practice progressed my mind has become more still and I have better focus on what I'm doing, but that doesn't mean "I'm perfect" every practice. There will still be days where I am sleepy, angry or hungry. Yoga isn't the practice of self judgement or perfect focus; it's the practice of self understanding, and learning to change your practice to a devotional and meditative act of breath and movement.
So celebrate the days when you're completely present and even those days when you are not. Every day on the mat is different.