You are just 19 years young chap. This is the time to feel amazingly strong and to have bubbling energy.  Consult a doctor and get a medical examination done to rule out any deficiencies and take supplements/medications. Get active with outdoor sports and/or hit a gym.  You will gain confidence and your self-esteem will boost that will bring you out of depression.  This would solve your lack of confidence,  stammering etc when you meet new people. Also seek help of a Counsellor and work on your emotions in case of any psychological challenges.