What you describe is by no means unusual, and is what is known as performance anxiety - or indeed, "stage fright" as you refer to it above. While I can appreciate it is very frustrating in the circumstances you describe, it is quite a common occurrence with first time sex and is basically down to the fact that it is a new experience for your body to be in. As such, the "fight or flight" reflex kicks in and your body floods the bloodstream with adrenaline and other such "stress hormones" - with the consequence that your blood goes everywhere except where you want it to in case you need to run away, and you encounter the problem you describe. In order to reduce the problem, therefore, you need to try and relax as much as possible and not let the excitement of the situation or fears about the size of your penis or lasting long enough etc enter your mind - and as much as anything, accept the fact that things may not go entirely to plan. You may headbutt the wall, kick a lamp off a bedside table or even fall off the bed completely (I nearly did the last one first time around...) - what's important is not to worry about it and not let it distract you from the main event, as it were. You also need to make sure you're fully prepared for it, and that means taking all the necessary precautions (condoms and other birth control methods), making sure both of you are comfortable with the thought of going all the way, and also keeping things in perspective. Your first time is always going to be a pretty significant event, but make sure it's going ahead for the right reasons and with the right person. Once you've got that sorted, the rest will follow. And if all else fails and you have a repeat of the problem, there's always something to be said for using your initiative, as it were.
Hope some of that helps.
