I'm going out on a limb here but I'm guessing that you can have an orgasm when you masturbate and that she is the first sex partner you have had or that you couldn't with the others either.
I think that you're problem is probably psychological. You focuss your energy to much on getting a climax and are worried about not getting one. When you have sex, your head is spinning like crazy thinking all kind off thing and having all kind of worries. In turn this means that you are not in the moment, you are thinking about the past worrieing about the future but not feeling in the now. You are not enjoying the sex in the way that you could and therefor you cannot orgasm.
That it is going on for a long time makes it even worse. you failed the last time so I have to preform which makes that you can't. So next time you feel even more pressure which makes it even worse and so on.
Now I wan't you to start getting back in the moment again. Being their with yourself and your partner. I want to ask you to try the following excresizes: You will sit accross each other on the bed naked. looking in each others eyes. You will take turns and tell something about the other you like. This will help you feel at ease with you being naked around her and it will let you know that she appreachiates you.
Then try a full body massage. This will help you two get intimate whith each other without any pressure of preforming. It also helps you two get accostumed with her touching your body and you touching hers.
You can try meditating. It helps you learn to empty your mind and be in touch with your body and your surroundings you will learn how to get in the moment.
Last when you do have sex take time to build it up. Take a long evening for it. start by cooking a meal. lite some candles and feed each other small bites of food. take a bath together kissing stroking but not the genital area. dry each other off and take her to the bedroom and give a massage there after that you can touch each other genitals and start making it happen for real. This builds up the sexual tension really high and helps you get ready for the intercourse part of it. It makes it easier to get an orgasm that way. Most important part here is that you don't go to when you are going to have sex but stay in the moment.
I hoped I could help you and that you can get pleasure out of sex (once again)
Oet Gäöl
P.S. I'm not a sexologist and if my advice doesn't work I suggest you get help there they can help you a lot better then any message board user can. take care
