Nisleis

I was looking through the forums and stumbled upon the old post by vivaceme on 10-24-06, and I have a similar problem right now, but I am younger... I have been dating my girlfriend for almost 6 months now, but recently in the past few weeks I haven't been able to orgasm during sex. The turning point was during a night a week or 2 ago where she noticed that I didn't go and brought it up... We didn't talk about it, just acknowledged that it had occured.. Problem is, ever since then I haven't been able to ejaculate (we've had sex 3-5 times since that night). I do masturbate still, and everything down there still works. I'm also not on any medication or drugs; and we've tried having sex with and without alcohol involved, both to same result of her orgasming, while I do not. Am I just putting too much pressure on myself over not being able to go and I just need to relax? Should I try to push for something different position wise? (Can a person actually getting "bored" with a position to that extreme? [All we do is missionary, and her on top facing me])