hi, when im with my girlfriend, i cant get a full on errection, only about 30%. i dont feel nervous or shy with her, and iv tried sleeping well before i see her. unless she directly stimulates my penis, then i dont get an erection, but even after stimulation, i only get 50-75% errection. this isnt hard enough to have sex. yet with previous girlfriends, iv had many full errections. i dont understand this, could you help explain?