That's what I thought zazen was supposed to be, but that's not how they're practicing it. When using the "counting your breath" approach, as I always had, they now insist that I start over whenever I hear something external, instead of of just noticing it quietly and going on counting, the way I always had. (Which works, and *does* create a certain amount of awareness.)
Their way, I never get to count past "1," and it drives me crazy! I eventually just get disheartened and stop.
Maybe you shouldn't count, then. I don't find it very helpful, myself. One problem with counting as a meditation device is that most people think of it as a process with a goal to get to some certain number -- the bigger the better, count to 10 and you win a prize, or something. But counting can become a very intentional, very conscious act too, particularly if you're trying to get to a certain number and thus see "starting over" as some kind of setback or failure (it isn't). So, don't count. Just keep your focus on the breath (a process that doesn't take any intention of yours; it happens by itself). Don't try to breathe a certain way, just observe yourself breathing (without criticism, "gee I am not breathing very deeply, better fix that" etc.). When your mind wanders, recognize that it's wandered and gently guide it back to the breath.
One thing that I've found helpful when it seems like I just can't keep from thinking about everything else, is to remind myself that breathing is all I have to do right now. I've set aside this time for this thing; this is my time to breathe, not to clean the house or get started on that big new project for work or send my sister email. I don't have to do any of those things now, and I also don't have to think about them or plan them. I have this time set aside for something else.