My did not erect! Why?

ok! I am 23 years old and I was on the brink of having with my girl in the back seat of my car, but my did not erect. This is the second time this has happened to me. What is going on? I notice that I was nervous and a little uneasy because it would have been our first time having in the car. She was sooo ready, but I was not! I feel very dissapointed and I know she is too but all I want to know is, is this normal? And to add to my question! This girl just so happen to be someone that I am only physically attracted to but not anything else. Do I need to pursue or just give it up? HELP!