One failure often sets guys up to mentally fail again and again. You can just start thinking and watching yourself too much "okay, I am kissing her....am I hard yet?.... its a little hard, so maybe I am going to get real hard this time....damn, shouldn't it be harder than that....gosh what is she thinking....she probably thinks I am too soft....damn now she is going to leave me, and I really like this one....c'mon get hard damn you.....now!"
Have you talked with her about it. If she likes you and is somewhat experienced she is probably understanding.
How much are you working, sleeping, training. Currently I work full time to support the family and go to school full time - often carrying unusual hours and I typically sleep 1 AM to 5 PM at which time I get up to do cardio. Between work and school I put in 60-90 hours per week. Sunday night I did not get to sleep until 2 AM, but decided to wake my wife up for some affection. I was fine to start but lost it about half way through for no reason. Just went limp. I have been married for well over a decade but still felt like a flop. She was okay with it, but I kicked myself for the next day or so. Luckily since then its been okay, but I am much more confortable around my wife than you probably are around this girl.
Lots of things can interefere with the process - fatigue and anxiety/stress are 2 of the most common. Watching your own reaction every second and judging your own performance or lack thereof I think is the worst. Like I said I have had it happen a few times and for me it works best when its happened to just fool around without paying attention or really trying - just enjoy tickling her back or something. If its happened a few times already she is obviously understanding and not going to up and leave you because of it. Jason Kidd had a few games where he just could not hit in this years NBA playoffs - everyone has a period where they have trioubles performing like they usually do, but keep playing and you will get past it.
I know alot of guys personal experience shows a blow job is one of the best ways to get a reluctant team member to stand up. Also some guys experience performance impotence - they don't get it up when they are with a girl they really like, but when they are tested for impotence they get erections and when evaluated with a penile Doppler it shows they have normal blood flow. They can use something like Viagra or Cialis for awhile, although they don't physically need it, but it gives them the confidence that "its going to work". Once they have been consistent and trust their team mate to run the plays, then they can drop the drugs and just let him get his game on.
Pardon the sports analogies.