Yea - those first few experiences really get to us.
But I wouldn't advise beating yourself up over your feelings or suddenly questioning the decision you and she have carefully thought & talked out - to the best of your current ability/experience.
It's absolutely normal and expected. She'll (likely) go through the same on you're first time "out" (if it's not together).
It roots from the distorted perspective we are conditioned with by society regarding sex. That 'programming' starts very early, when our brains are very much in their formative stages, and won't be undone just by a conversation and decision.
That make sense ?
It does take some work. Yeppers - there's a little pain involved. But it DOES pass, and usually faster than we might think. That fact that you two seem so close and have had the foresight to discuss where sex played into your relationship in advance only makes it easier. Good for you !
It's also good to remember that 'sex' is just 'sex'. The sex we partake in with different people is different because the equation is different. What you have together that's good (or great) won't change because of it. And at the heart of it all - it's NOT what defines and glues your relationship together ! (I hope)
Hope that helps some. Most here have been there with you before.