At certain point of meditation,I feel like I can't breathe properly as in the beginning of meditation(lose of oxygen) and thus I stop meditating and try to get oxygen into my brain. I do no know why this is happening or how to overcome this condition. Is that not a correct way of meditating? Or what am I doing is wrong ?Or do I have to push through?
It is difficult to answer your question due to not many facts. However, the impression is you are suppressing your mind or pushing your forehead down into your body; making your mind & forehead/forebrain rigidly still; and, at the same time, pushing your air/breath/outbreathes out; clenching your jaw & emptying the lungs & nostrils of breath.
In Buddhist meditation, the face should be relaxed and the mind/brain bright, awake & open. The Buddha taught to start meditation by establishing awareness in front of one's face. Possibly do some practise keeping your eyes gently open and keeping your face & jaw relaxed. Buddhist meditation is the practise of "non-attachment" rather than forceful suppression.
It is probably best to visit a Buddhist/meditation centre and talk to a teacher.
Just before the nimitta appears, a lot of yogis encounter difficulties. Mostly they find that the breath becomes very subtle and unclear; they may think the breath has stopped. If this happens, you should keep your awareness where you last noticed the breath, and wait for it there.
A dead person, a foetus in the womb, a drowned person, an unconscious person, a person in the fourth jhāna, a person in the attainment of cessation (nirodha·samāpatti), and a brahmā: only these seven types of person do not breathe. Reflect on the fact that you are not one of them, that you are in reality breathing, and that it is just your mindfulness which is not strong enough for you to be aware of the breath.
When it is subtle, you should not make the breath more obvious, as the effort will cause agitation, and your concentration will not develop. Just be aware of the breath as it is, and if it is not clear, simply wait for it where you last noticed it. You will find that, as you apply your mindfulness and wisdom in this way, the breath will reappear.
Knowing and Seeing (Fourth Revised Edition) Pa-Auk Tawya Sayadaw
How do i avoid this and not lose concentration
Some instructions suggest that concentration is developed gradually -- for example the article Calm Abiding describes three stages, starting "with external support" and then "internal support" etc.
Instructions don't tell you to stop breathing.
Some instructions tell you to be aware of the breath -- notice whether the breath is long or short.
at a certain point in meditation i feel like i cant breathe (lose of oxygen)
If I ever feel I can't breathe or "I forget to breathe" then I breathe out (exhale, blow out) -- after that the body will breathe in.
I think that the body is meant to be able to breathe by itself i.e. without "my" help, it's that "I" shouldn't try to stop that.
It sounds like you might be attaching to a specific point in the breathing cycle and wanting to prolong that moment, instead perhaps you should allow that point to happen again and again as the breathing cycles -- i.e. "continually" (again and again) not "continuously".
It's impossible to lose consciousness through meditation in the way you're describing. Your reptilian brain simply won't allow you to not breath for any length of time. Just let go. Don't force your breath. Your body is going to take what air it needs.
What you are running into right now is a mental obstacle, not a physical one. Your body/mind is trying to act - trying to stay in control. Don't let that happen. Don't be taken aback if the next few breaths are sharp or jagged. If you let go, they will taper off into more subtle inhalations.
I'd also second dhammadhatu's answer - you might be slouching. Make sure your body is erect and relaxed. Imagine someone hold you up by a loop that's attached to the top of your head. The body just kinda hangs from that point. Everything is in a perfect line from the top of your head to your coccyx.