i think it's just "performance anxiety". you should relax. maybe you're trying to control things too much. i had a b/f who had this happen to him a couple of times. he was just psyching himself out.
if you're "concerned" about what you're doing, you just have to decide whether it's right for you and her. if it is (right girl, right time, etc.), then let go of your concerns a bit. i'm not trying to encourage you to do anything stupid, but i'm just saying that you should feel good about what you're doing, and if you are nervous about it or anxious about it, then that's not going to help you. in the case of nervousness or anxiety, either let go of what's bugging you, or just don't have sex until this is no longer an issue in the equation.
if "concerns are not your problem", then here's what you do. forget about your penis. just focus on having a good time. let her work with YOU too! think about it. you do foreplay on her to get her all ready to go...let her do some on you. don't have sex with her when you're in a hurry. take your time and who cares if it happens right now or 1/2 an hour from now? see my point? what will happen when you adopt this kind of attitude is that you will both have a much better time and you'll be able to let go of all this stress.
99.9% chance there's nothing wrong with you, you just need to relax and enjoy yourself more, take the pressure off, and just play the game. this is particularly difficult the first time you have sex or the first time you have sex with a new girl. it's common, so don't worry about it.
maybe just don't "plan" sex. as long as both of you agree that you don't have a problem with it, why don't you just let it happen when it feels "natural" to you. take the pressure off...don't push yourself. this is going to be one of the most fantastic times in your life, so instead of letting it be a let-down, or someting that is embarrassing, MAKE it awesome and fun for both of you! good luck!
jasmine