im 17 years old. me an my girl friend have tried to have sex twice but the first time i wasnt able to keep my erection.i have had sex before an this never happend. the second time we tried it happend again my girlfriennd was kinda mad an now she thinks its because of her an im not attracted to her. but its not i am very attracted to her.i dont want this to happen again. what should i do what should i tell her?