I have been breathing from my mouth since ages and now when I try to concentrate during my meditation with the breathing techniques, Haung Saw, deep relaxation, tense and relax, etc I feel tired and am unable to concentrate and thereby it affects my meditation. Moreover during the day I am too engrossed in my work to keep a check on my breathing pattern and consciously change it to breathing from nose.
Please suggest what should I do? I would really appreciate if you can help me out.
The value and importance of natural (nasal) breathing cannot be over emphasized for balance and health of body, mind, and spirit. But that having been said, a lifetime of mouth breathing may take you some sustained and patient effort to re-direct.
Quite possibly the experience of fatigue from nasal breathing may be due to not getting enough oxygen. Obviously with mouth breathing you can take in quantities of the "stuff" more readily. To counter what might seem a loss of oxygen, it will be important to activate the use of the diaphragm: the gentle but natural expansion of the large muscle below the ribcage and over the stomach area. As we intake breath with this natural movement, a secondary lift comes as the floating ribs expand and rise, and then finally as the upper part of the chest lifts just slightly.
You might also consciously (however slightly) compress the lips before and during practicing natural (nasal) and diaphragmatic breathing. Do this technique not just as part of meditation but whenever you have a spare moment, on a bus, reading, lying in bed and so on.
Also, learn to relax the nose (especially) on the inside, to expand the flow of air. Before practicing Hong Sau, work the natural breath for some ten minutes or so (even longer if you like) using breath control and this natural breathing technique.
Hopefully you do not have some obstruction in the nasal passages that has perhaps been the cause of your long standing habit of mouth breathing. For that you might need a medical examination (ear, throat, and nose doctor, perhaps). Try to feel whether the inflow and exhalation through the nose is approximately equal in strength when you are practicing pranayama.
Be patient; it may take some time and lots of practice but it is well worth it. Natural nasal breathing may help your resistance to colds and flu, improve clarity of mind and other mental functions, and may even help you smile more!