Thinking about orgasming and wanting it so much in the moment to happen, will just make it not happen you are to much in your head then...
Sort of like a watched pot never boils.
You need to find or do something to relax you before having sex.. even if having a drink or two.. having him massage you first, lots of foreplay to relax and ready you.
You need to first come to terms with your own body. I would suggest Susan Hoopers comprehensive book. It is one of the best I have seen for men and women.
What are you thinking about when you have sex? Are you comfortable with yourself? These are a couple things that could affect that. Someone who is very self concious about themselves could have a hard time with that because all they can think about when having sex is, "Do I look sexy enough?". "Am I doing everything right?", stuff like that. I've always had a problem with having a orgasm with someone I just met cause I wasn't comfortable with the situation. But after I really know someone, I can feel more comfortable cause I know what they think of me.
i guess it's just frustrating, you know?... the guy i'm with now has spent a good bit of time trying to help me out with my little problem, but even with everything he does, it doesn't seem to help...
You need someone that you trust and care about who is patient and knows what to do to help you. One key is to be able to relax and clear your mind. The otheris to stop trying so hard. One trick that I learned over the years is to "squeeze" yourself using the keagle muscles.