How To Increase The Size Of Penis?

hi, i am 18 years old and i am very uncomfortable with the size of my penis, it is around 1 inch flaccid and about 3-4 inches erect, I have been with the girl of my dreams for 3 and a half years now and everytime we have sex i lose concentration because i feel uncomfortable with my penis size. I am wondering how to increase my penis and if my doctor can prescribe me with a penis growth pill of any kind?