I always recommend that my male clients see their general doctor first. Get a full medical exam, including checking your testosterone levels and prostate function as well as your cardiovascular functioning and blood sugar. Then, take a look at any medications or supplements you might be taking. Do you ride a bike? Does that bike seat have a nose? All of these factors need to be considered for ruling out where the issue could be stemming from.
How’s your masturbation? Do you have difficulties getting and maintaining your erection when it’s a solo show? You could, as scary as it may be, talk to your girlfriend about the issue. See if she can help. Make it a couple’s endeavor to work with you instead of being scared.
Part of your performance anxiety may be stemming from trying to hide this from her. Take sex back to the basics – enjoy yourself and have fun. Make out. Do more oral. Try out fantasies where penetrative sex isn’t the focus, but a bonus if it happens. Find ways to lighten your perceived pressure to “perform” a certain way. Redefine what it means to be intimate and have sex with your girl. When your erection and climax are no longer the highlighted feature, but the addition, you might just find that you are able to achieve and maintain erection after all. If all physical components are ruled out, and nothing else is helping, then I would venture to suggest maybe consider getting in touch with a sex therapist in your area. He or she can help uncover what is going on and how to help you work on overcoming it. I can understand that seeing a professional is anxiety producing. I want to assure you that it can be profoundly beneficial to you in the long run.