i cant gain weight

Im 15 years old, and i way 95 pounds, and im around 5'4. I hate my figure, and want to gain weight. I eat so much a day, but nothing seems to work. I tried doing some excercises, thinking maybe i can get my muscels on my legs to make them bigger, but that didnt help either. I checked up with my doctor and they said that i am under weight but i am healthy. But i can not stand looking at myself in the mirror, i get discusted with what i see.Is there a reason why i am made like this? Does anyone have any other sugestion on what i can do? If theres any pills that may help? if theres anything in particular i can eat? any excercises i can do to help me?
Please someone give me an answer. Thankyou