yeah...its happened to all of us, mostly becuase you care about this girl and you care what she thinks because she matters to you. you want her to know that your a machine in the bedroom that would be the ideal situation but we all have trouble doing this.
advice from me that ive used to get over this
take horny goates weed...some pill u ccan get from any supermarked in the condom section(for fun, its full of caffine and blood flow stimulating herbs and gets you horny and hard all the fucking time, i was riding the bus home and it was a bit bumpy haha

)
or alcohol works me, ups the arrousel levels for me and you can block out thought for a while and let the good times roll (just not tooo many drinks lol)
BUT, then despite doing this i still sometimes randomly have problems as soon as i put on condoms, (then constrict the blood out) hey it happens, dont be ashamed of it lol, just make a game of it.....Roger weve hit a problem over, in need of a blow job how do you copy, over...then forplay for a while and make sure tht the environment your in is warm, ive heard when its cold it can lower arrousel levels by a LOT so turn on the heater or light a few douzen candles. this is where i think alcohol finds its way in tricking your body about the temperature
hope this helps in any way man....after a while you will stop caring and can start pleasing your woman.
Ash