I think it may be performance anxiety, but for what ever reason it is hard for me to get turned on around my new girlfriend

I am only 19. With any other girl I have been with it was no problem, I had total control. I could either last all night or cum on command if I had to, I had it down to a science. But my new girlfriend, I am absolutely in love with, and she is the hottest girl I have ever been with. I was wondering if that could be a factor? Like, maybe I'm nervous? When we have sex I can get her off 2/3 times at a time, before I even go once. I was wondering if it was the condom, but even when I'm not wearing one, it is still hard to get up some time, and unless I am constantly being stimulated (sometimes I can lose it in as quick a time as switching positions) I lose it. It's tough because she thinks it's her fault, but it's not and I can't seem to figure it out or get over it. Any idea what it could be? I sometimes have to fake an orgasm (which is something you usually hear the girl say) just so she thinks she is getting me off, because I can seriously go for hours.