I started with a new goal -meditate for a month daily and it reminds me of my experiences which I was experienced during group meditations in the past. After relaxing the body it gets very hard to breathe, my chest feels very heavy or like something is on top with a lot of pressure. I feel like I should breathing deeply all the time but even in that case, it's not enough air. A bit scary too. Did anybody experience it too?
it's not enough air. A bit scary too.
It's quite difficult to give detailed advice over the internet. What you describe is actually somewhat concerning. The best advice would be to seek out someone who could watch your meditation and advise.
For example, a common reaction to fear can be to hold one's breath so that a predator cannot detect our breathing. Meditation should feel relaxed and secure and peaceful, especially if you are alone without support. Simple things like poor posture can magnify suppressed anxiety. For example, if my chin is sticking out a bit, then fear tends to arise. However, if I am sitting straight and relaxed, chin in and back of neck smooth and unkinked, then I feel secure and fear evaporates.
Please do seek at least a friend who can compare your sitting posture with a picture of a meditating teacher. Such a friend would be a good spiritual companion.
Rationally I know that when I'm bicycling (or otherwise exercising, walking, climbing stairs) then the breathing then (when there's a need for an increase flow of air) is sufficient to keep the body alive. So there's no need to be scared of that (insufficient flow of air) when you're just sitting still -- the body can take care of itself, breathing is autonomic (see also apnea).
Rationally too I know the body will die of something -- possibly (or even always) insufficient breathing -- so might as well get used to that (e.g. note the perception or feeling with equanimity) and not scared (maybe become conscious of and let go of whatever "attachments" -- some fear of "dying" for example -- might make you crave the situation to be otherwise and cause suffering).
Perhaps the feeling (of insufficient breathing, if this is what you're feeling) is a result of too much CO2 (i.e. carbon dioxide), rather than too little oxygen. I think people can get confused when that happens and try to breathe in more, to inhale more deeply -- whereas actually what the body needs is to breathe out more, to exhale. See also e.g. a "barrel chest" being a symptom of emphysema -- when the patient can breathe in but can't breathe out properly.
I find that breathing out (exhaling) properly/deeply/completely, using the ribcage and the diaphragm to empty the lungs, is a prelude to (i.e. a start to) a slow deep breathing cycle, and a component of it.
Excellent! That's a very, very good sign. It's indicative that you are starting to loosen your conscious hold on the breath and, instead, are allowing yourself to be "breathed". Like anything else, there's always some fear associated when we relinquish control. How much more so when that happens with the very thing keeping us alive!
Buddhism, more than anything, is about trust. This is your first opportunity to really experience that. Trust that your body knows what it's doing. Forget about inhaling and exhaling. Don't concern yourself with deep breaths or shallow breaths. Just let breathing be what it wants to be. If you give yourself over to the process, I guarantee that you will finally begin to know what breathing is all about!
In my experience leading meditation groups with folks who have anxiety, panic...focusing on the breath during meditation can trigger the physical feelings of panic if the meditator is overly sensitive to physical sensations in the body. This can happen in groups when the leader is pacing the breath too fast for you or you are breathing too deep, causing too much oxygen/too much CO2 which creates symptoms like lightheadness, pressure, panic etc. When you are new to meditation and are susceptible to this, it is important to start with 'noticing' your breath vs. trying to alter it. You could accompany 'noticing' the breath with 'greeting' any of these symptoms as they appear, "hello chest pressure, thanks for giving me information today," and then let the thought subside. You could also try some other forms of meditation (walking meditation, guided meditation, body scan etc) to kick off your practice and then over time add in more advanced breathing techniques.