"The key is to focus on your breathing, and pay as much attention to it as possible. As other thoughts enter your mind, recognize that this has happened, notice the process. If you must, you can even examine the particular thought for a brief moment (what type of thought it is, why it may have popped into your mind), but always bring your attention back to your breath.Think about it this way, if you are consciously telling your brain to focus on your breath, but you get distracted by something, you aren’t really in control, are you? That’s your subconscious injecting a thought into your conscious that it thinks is more important.
Losing concentration on your breath (and you will), and bringing it back to your breath, is exactly what meditation is. It’s strengthening your mind’s ability to focus. It’s like lifting weights for the brain; each time you lose focus and bring it back is one rep. The longer you can hold your focus on your breath, the stronger you are becoming. And yes, it can get mentally tiring! But the next time you are in a tense or stressful situation, your stronger mind will be able to handle it much better. One other important thing to note is that just because you lose focus doesn’t mean you aren’t meditating or that you are failing. That’s part of the process. That’d be like saying just because you can’t continually lift a 30lb dumbbell over and over again you are failing at working out."