PIED

I have been watching porn since I was in the fourth grade and never actually experienced being with an actual person before even though I am 19. Was talking to this girl for 3 months and we have never even attempted sex until a few days ago. Whenever we kissed though I would get an erection but in the moment of us actually about to do it for real my dick could not get hard for as hard as I tried. Can anyone give me some advice on what is going on and what I can do so that this will never happen again.