I am absolutely sure this is because you worry. For the first time, every man wants to make the best impressions possible. This is why we always worry. Having no erection for the first time with a girl is a terrible thing. It can ruin your whole relationship. I had that thing with a girl whom I was dating for a while. After the failure, I broke my relationship with her because of the shame. I think you can find some of the reasons why your erection is bad in this article
4 Situations When Viagra is Not Able to Help You