Hey man....I have the same problem as well. It's been bugging the living crap outta me for the longest time, this caca has been going on for almost a month and it's killing me.
I've had sex with her before but I noticed it happened one night when we were going to have sex. We went through with the foreplay, I had an erection, and as soon as the pants came off I lost my erection. It's embarassing as hell and it really sucks that I can't perform. Then I have to make up some crappy excuse to my girl.
I've been told a few things...Stress, anxiety, but I don't know. I think about it so much then I get all worked up about it and crap when it happens again. If anyone knows how to solve this problem, please, let me know...Thanks.