Hi I am 18 years old. I do not drink ,exercise daily and have no other problems. But when I try to have sex with my girlfriend I get hard when we are kissing and all. But when we started to have sex I couldn't get it up. I have no problem in masturbating. And wearing condom was difficult for me for the first time because it was the first time that I got to know that the head of the penis is exposed ,before that I never retracted my foreskin. The head of the penis is also a bit sensitive. Can the problem of my erection be related to this? Please help.