As far as stammering is concerned, I would like to suggest you to do self-practice of 40-45 minutes daily as follows; but before going ahead, commit to yourself to go the practice with proper dedication and punctuality; only then it wil give results:
(i) Identify those alphabets of English and your regional or national language; remember I wrote about alphabets- first identify alphabets which you find difficult to speak or pronounce. Introspect yourself and meditate on your alphabets - this will let you find your weak points in communications. Find yourself the ways how can you pronounce those alphabets easily with alternative articulations. If you find yourself difficult, write to me with those alphabets; I will try to help you regarding those alphabets to pronounce or speak with different style.
(ii) Identify those words in English as well as regional languages. Then pronounce loudly in isolated place - many times- remember you have to give 15 minutes daily. Not a single minute can be relaxed for this practice - only then you would be able to improve yourself. If you have more than 15-20 words, you can make your time table - to practice one set of 10 words on one day and other set of other 10 words on alternate days. This way you would be able to practice good in a week.
(iii) Speak loudly the newspapers of your language in isolated place, if possible in front of the mirror seeing yourself and your expressions of self-confidence. Again train yourself with this practice with 20 minutes.
I have been confronted with cases of stammering, most of the same are not due to any anatomical or physiological defects rather they are due to lack of their moral strength. Accordingly, I have been suggesting them to go through the techniques as are prescribed in personality development courses or in literature books.
To become overconsciousness while interacting with strangers and even in interviews, i think, it is obvious to get a little bit heart pump especially, when there is a situation where your talents / calibers/performance are being measured/ judged. At that time, you can try to breath in and breathe out in ratio of 1:2 as per your comfort level.
I think it is not necessary for you to let you know that when the speed of thoughts and vocal speed does not match, stammering is produced. So, I would suggest you to go through some yoga techniques /meditation /progressive relaxation techniques that can teach you to relax/calm your mind. In the same line, go through some personality development courses or read some literatures, literatures (books and others on yoga/ personality development / meditation, etc.) makes person mature and practical and let him rid of illusions, etc.
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Padmasana (with gyan mudra)
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Simhasana
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Naadi-shodhana pranayama
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Suryabedi pranayama (in cold season only)
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Bhastrika pranayama
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21 times Om Chanting
Deep Relaxation Techniques or Progressive relaxation (or shavasana)