hello doctor.when i had my first sexual intercourse ,my penis was weak.i was mastubating frequently by that time.later i had to take viagra tablet to please my girl in bed eventhough it was not frequent.i am 23years old and presently if i want to have sext,i erect well , when i take off my dresses to penetrate ,my penis become weak.later , some time it erects when i want to penetrate, i become weak.please doctor i have lost many girls as a result of this problem.i want you to help me.