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Gastric issues most of the time are related to lifestyle like odd hours of food ;no exercise;eating more of outside food etc;so better make changes in your habits if there is a problem.If you have acidity take medicine course prescribed by a doctor .Mental tiredness
can be due to mental stress;family issues;over work ;work pressure.Even dandruff
is caused by stress both physical and mental and and you are underweight
also.I would suggest you to get examined and investigated to find the physical cause of being underweight and get treated by a doctor.Once the cause is known and treated you should be fine.The confidence isalso related to physical wellbeing so better get treated .Start on a multivitamin
and you should feel better as vitamins
like B12 deficiency are known to cause depression;fatigue;weakness and even loss of concentration.