Can you ejaculate on your own? If you can then most likely problem is performance anxiety. I know the word 'anxiety' might be shocking. But really its just a word in reference to the body's reaction to stress hormones. When you get excited, when you want to have sex and please your partner, you release stress hormones. And these can have an impact on your sexual performance.
You can try to have a session of oral sex first. If she can get you close to climax before you have sex that could help you out. Or if not a bj, try a hand job. It might even help you warm yourself up in the bathroom before sex.
You might also try a performance enhancement product I know you can buy some different things at walgreens. I think they are produced by GMC, so quality products. But they may make it easier for you to orgasm. I know sometimes when I just have a cup of expresso or something like that, the caffeine can intensify sex.
If that doesn't work then you need to see a urologist to make sure you do not have any serious health problems.
