Ok i have had sometimes where i got hard but lost it with a ex girlfriend. And it was disappointing and embarrasing. I really think i was over anxious and thats sort of the reason. But that memory has stuck with me alot and now sometimes when im about to have sex i think to myself, dont go down,dont go down and well u know the rest. I really starting to think lack in self confidence is part of my problem, some with health too but mainly its in my mind.
every guy has or will have that moment, it sucks. don't let it ruin your mood though, just go into foreplay, go down on her. trust me, you'll get hard again once your mind is forcused on her body instead of your cock. solid!