Do you have these problems only when your with the girl? If you whatch porn by yourself, could you hold an erection since the pressure of the girl being there is gone?
Im asking this because if in both cases you have problems you might be suffering from a T defficiency.
If not, then its just a confidence issue and the supps might not help. I was intimidated at first too and had a hard time because the girl i was with intimidated the shit out of me she was experienced and older and i wasnt. But once i got in there shit was a breeze. Youll get over it, you just gotta conquer your fear, and some puss.