Well obviously I am not a guy, but I have been with several boyfreinds and they have had similar problems,
the questions I qould ask you is:
is she your first partner?
Were u a virgin before u slept together>
and is she more experienced than you pehaps?
Coz if any of them are true it could be nerves, nerves do cruel cruel things to men, maybe u r worried u wont satisfy her, or u wont be good enough?
Coz trust me that can make a man lose his errection.
The only way it wil get better if this is the case is if u feel comfotable around her and she makes u feel relaxed if anythin does go wrong, ina relationship these things happen but its best to laugh it off and hey practice makes perfect
i very much doubt that u have a medical condition, in most cases anways it is a psychological factor behind it.
If it was impotance then you wouldnt be able to get an errection atall.
How about not go for intercourse, and practice on your foreplay and just do that for ages, it is hard for even an experience guy to bang away for hours!
So relax! And I hope I helped
k x