Why Does One Have An Elongated Penis?

my penis is almost 11 1/2" long when erect, and although my friends tell me i should be happy with it, it sometimes scares or hurts women during sex. I realise that penises can vary from very small to very long but i don't think i've ever met another man with a penis as long as mine. I would like to know how common it is to have a penis this long as i'm worried it will prevent me from having a proper relationship.