hello, i am not happy with my girl friend, when i go for sex my penis comes strong very quickly & it comes out very short times(within 2 minutes). after that when i want to go for another time i feel very tired and penis not come strong same like before, i can not success in the 2nd time always. now, how can i improve my sex power, how can i stay for long time, how can make 2nd time sex possible.