Do i have ed or can this be treated?

I am 19 about 120 pounds. I have a problem when haveing with my gf, don't know y but when haveing I can not fully erect or during my dies down. Not think its ed because I can stay hard when ing. Dnt know if its because I to much or need some sort of vitamin. I want to know why I can oly stay hard only when I and can't get hard wit my gf (by the way beautiful girl so she not the problem)