Anna, and are your shoulders unfolded? If slouching, it can be difficult, the chest is not open.
In theory, inhalation and exhalation are completely natural in this process, just like in ordinary life. I have not come across such a thing, I myself meditate - freely. The only moment is the position of the body in space. Try to raise your shoulders, pull back and down a little, connecting the shoulder blades, then the chest will open. And yes, don't pinch your stomach.
It should work.
I'm not a meditation master, but the obvious answer is to change your posture to be comfortable. As far as I know, there is no super-rigid posture when meditating. There is no such covenant from Buddha that one who does not sit in a perfect lotus will not attain enlightenment. Otherwise, it turns out that there will be semi-enlightenment in the semi-lotus? Will the quarter lotus have a quarter enlightenment? And if you sit on a chair or in a chair, and even lean against the back of the chair, then you will not see enlightenment at all? And what about people with stiff legs or a crooked spine?
So, as far as I know (and as far as elementary common sense suggests), the lotus is a wish, not an absolute requirement. Simply, provided there is sufficient flexibility and health, for a person of the Eastern culture, in which it is customary to sit on the floor, and people retain flexibility of their legs since childhood, a lotus or half lotus is the optimal position. They sit in it not because it is right, but because it is convenient. Europeans, who are not accustomed to the lotus, should sit in a comfortable position as close to it as possible (half-lotus, on their knees or even on a chair). There may be some inconvenience (to somehow get closer to the lotus), but meditation should not turn into a continuous excruciating struggle with pain and discomfort.
This is if you are tied to Buddhism, enlightenment and the like.
If the goal is just to collect your thoughts, calm down, pause and look at the flow of life from the side, then the posture is not so important at all. For example, I meditate for 20-30 minutes almost every day in general, just sitting on a chair with a normal back and watching the breath, without breathing control and special technique. In order to get out of a state of stress, think about a task, reduce anxiety and sort things out on the shelves - it's enough. But I'm not a Zen master)))
I am 16 years old