I'm almost certain is nerves mac, try to calm down, relax and enjoy it. Let your dick do the thinking when your undies are round your ankles.
If nerves are going to effect you then my advice is go back to the same one, you know her now and you know she's genuine and will give you a good time.
I can't help with how I felt on my first punt, it was over twenty years ago, it was a blow job from a lass in Bradford. I must have been nervous I guess but can't remember, I shot my load, I do remember that ha ha, so must have been hard.
Don't be getting caught out with this tactical wank bollocks, go bagged up and ready to shag owt with a pulse, you need this first one out of the way and successful to give you the confidence for the next one, you'll be fine then, stop worrying.