Sometimes I forget to focus on breath while practicing yoga. In fact for the longest time I did not realize how important it is to actively breathe. I was just there for the workout.
Active breathing is fuel for your practice. Without it the practice becomes labored and difficult. Without your breath, you miss out many of the mental benefits of yoga.
Next time you are on your mat, make your breath your focus. Try to follow the breath queues from the instructor. If you forget to maintain the focus on it, don’t judge yourself, just simply go back to your breath focus. Continue with this until the final “savasana” and perhaps try to stay in a longer savasana.
Give it a try. Then notice how much energy you get from it. Breath is energy and it helps you have a better more sophisticated and meditative practice. In the beginning you will have to consciously focus, but eventually it will become an automatic part of your yoga journey.