Man, don't be so hard on yourself, these things DO happen. Also, if she's the first girl you're having sex with after having employed your PU skills, it tends to give a different feeling to things. You are
half thinking about wanting to have sex with her and half thinking about what you should be doing next, doing freeze-outs and stuff and running over stuff advice people may have given you that pops into your head and that just adds to the nervousness. I mean, I could be wrong, but from how you wrote it, it sounds like you were kinda in that mindset to me and that can definitely be distracting, so you've got to just live in the moment and enjoy it.

Also, believe it or not, women are really quite understanding, I mean, they don't have to DO anything in order to be capable of having sex, so the fact that we have something that has to be functional, they seem to get that if it doesn't work, it isn't exactly like a light switch (at least this is my experience and I've experienced this...more than I'd like to have, due to being diabetic and having terrible blood circulation). Here's the thing, realise that she's understanding of this, don't worry about it, give yourself a break and next time try not to think about it, just enjoy yourself, cause you know she did the other night and she will again.