Hey Doctor I Am 19 Years Old Male African American

Hey Doctor I am 19 years old male African American and I like to have sex with my girlfriend but I masterbate a lot when I'm with other girl though my penis doesn't get hard why is that? When I masterbate it gets hard and stay hard!! But anyway That lead me to be scared to have sex with any girl because I don't want them to laugh at me. I'm up to the point where I think I should take vigar please help