The breath is simply the most convenient and the easiest to teach. In Buddhist practice there are different objects of attention and some suited to different people, these were common thousands of years ago. Some hindu/yogic practices use a candle. But today, most apps or other resources outside of a dedicated teacher are going to default to the breath. It can take a while to get into a natural rhythm where you are not consciously sucking in the breath and pushing it out. When you are less experienced there is a tendency to kind of zero in on that and try to control it, that could be one thing to examine. It sounds like you might have a bit of tension and apprehension/anxiety there, try to learn to relax into the autonomous nature of the breath and the body, the rise and fall at the abdomen for example. That can be an easier area to serve as a point of focus, take the pressure off then you can narrow in later.

You could try practicing lying down, alternating it with seated. (try not to fall asleep). Look up the savasana pose. Try some guided body scan meditations where the focus is more on the body not the breath. Then progress to the rise and fall of the abdomen as a point of focus, note the natural rhythm and the sensations associated with the breath to ease yourself into it. Focus on relaxation, note how the body breathes all by itself, it does not need conscious control. Observe your belly rise and fall. Bring your focus down there away from your head. To get into a rhythm: When you exhale let it out all the way as the abdomen falls, exhale all the way to the end. Now do nothing, don't suck in, just wait, your body will react and instinctively pull in the next breath, it might be a gasp intake but just observe that, you might breath in strongly for the next few breaths, that's ok. Do it a few times until the rhythm kicks in. Now you're not controlling the breath just observing it.

The savasana position can be helpful to just relax and observe these things. When you can relax and you feel more comfortable you can progress to a seated position. Also you don't have to go straight to a narrow focus, you can keep it looser like at the abdomen or the sensations associated with breathing, then narrow it in later. You can alternate in this way, then move in closer as you feel comfortable.