I don't give girlfriend sex

Hi good evening. I need help. This is very contradicting. I've been with my girlfriend 4 years and she's the best I find her sexy she beautiful both inside and out. I have fantasies all the time about her. She tick all and more of my boxes. I love her like nothing else. Now to the contradicting part sex!! I want sex all the time right up to sex then for some reason I get anxious about it and not bothered. It's been like this from start. I'm confused. Now my girlfriend is not happy. But in don't. Know what to do. I've told her its down to my penis not working but that's not true it works fine. Its not been like this in previous relationships.