Remember that no matter which practice you choose, there will always be times when your mind wanders and you lose track of your breathing altogether for a while, especially when you first start trying to follow your breathing, but even if you've doing it for years.
And whenever you notice your mind is no longer on your breath, it doesn't mean you've failed or done anything wrong. That's perfectly normal and perfectly fine, and simply acknowledge to yourself that your mind has wandered and gently return your attention to your breathing and resume counting or measuring or however you were following your breath.
If you've purchased the course, you can download the complete set of audio files organized by lesson here.