This is common misconception about mindfulness meditation. The goal of mindfulness meditation is not to quiet mind(though this happens with practice). It's to learn how to notice when your mind is unquiet.
So when you notice your mind wandering this is mindfulness. Imagine you're coding all day long, and then you have this moment where you notice that you're not paying attention to coding, but instead thinking about what it would be like to be a ninja. You might have been thinking about being a ninja for 5-10 minutes before you realized it. But this moment when you realize, oh hey I was thinking about something other that what I thought I was thinking about is a moment of mindfulness. And this is what you're practicing.
So when you notice your mind wandering, be happy, this is a moment of mindfulness. Your mind was probably wandering for a while before you realized it, but this moment is the moment you realized it.