This all sounds like it's in your head mostly. Putting high expectations on yourself to perform. PE Training yourself to perform a certain way and when you finally get the pussy and it didn't workout as expected. It all came crashing down as a failure. Well, it's not, it's just told you what doesn't work for you.
@the boss! is probably correct. Your unit is fatigued from your PE training and couldn't stand up to your expectations when it was time to perfrom. As he recommended, take a couple days off from PE training to reenergized your unit.
When it comes to sex, enjoy the person. It's not about busting nuts and taking names; that's the mentality of �naked people movies�. It's about trust and making a initmate connection with the person. Stop worrying about how well you're performing. Women care more about the attentiveness at first than jumping straight to banging the shit out of her; only sluts do that.
Do prolong foreplay, kiss her, make out with, explore her body with a soft caress all over, fondled her tits, rub her ass, play with her pussy ask her does she like her tits rubbed, her ass, her hair pulled, whatever you're doing to her. The ironic thing is, she is going through the same shit you are, worrying and nervous about her performance. Thoughts running through her head, is she thin enough, my ass or tits big enough, is she wet enough, his dick isn't hard, don't I arouse him.
Sex starts between the ears way before it starts between the legs. Get your mind (big head) set first before you dive in with your little head. With 15-20 minutes of foreplay, both of you will be ready to fuck each other brains out.
Leave the viagra alone, it will eventually become a psychologoical crutch for you to have sex.