What Medicine Can I Take For My Weak Penis ?

Hi,i am student,bechelor from Bangladesh.weight 50 k.g,height 5.8 inch,nd 21 years old.i ve been musterbating almost 5 years.nowadays my penis doesn t get harder like before during sexual intercourse .if anyhow penis gets harder,becomes soft after 4-5 seconds later.i ve not taken any medicines for this problem.so,can t give pleasure to my girlfriend like before.please suggest me what kind of medicines and food have to take?