Near exams or difficult /risky presentations, i get upset stomach&pain, and get disturbed sleep, please help me as to how can i prevent this?

Stage fright: You are nervous- that is why you have those upset stomach and sleep disturbance. If those symptoms are not affecting your job/school significantly - it is a normal reaction. However if it is so severe that you can not function and you are just gonna blank out and freeze- then you will need help. Discuss with your doctor-medication and behaviour therapy can help. A very good preparation will help.

See your doctor: It sounds like anxiety. Your docotr should be able to give you something for this, if appropriate.