This problem sucks. It happened to me earlier this year with a chick that was my roomate. She was also a huge bitch, but she was hot as shit...I'm talking HB9 ATLEAST. Anyways, the first time I tried to stick it to her I had been smoking weed all day and been fappin it quite often. I never had this problem before so it freaked me the hell out. But once I found out that usings lots of drugs and frequent masturbation was probably the problem. I cut it out quick! But there was also another factor, SHE WAS A BITCH! This is what would happen, once she saw I couldn't get it up, she would kick me to the side of the bed, pull out her vibrator, and make me watch her please herself with a machine that could do what I couldn't! She would also bitch at me constantly during sex. But one night I got her to stfu and eventually got the old gun up and started firing away. After that, it was cake.

But another thing also happened, once I stopped beating off all together, I went to fuck her one night, and I literally creamed myself before even taking my pants off. So there was not enough masturbation? IDK!!!

All this together literally fucked with me for awhile.

So 3 BIG things to note if this is happening to you: 1) make sure your comfortable with this chick and feel no pressure, if she is bitching at you in the sack it's going to fuck you up!
2) make sure your drug and alcohol use has been very limited in the past day or two.
3) same rule applies for masturbation.

Stick by these three and you'll be all good!