Changing positions, adding in role-play or props or mechanical devices, any or all may help for a while. In the long run, though, I'm afraid that you are one of those novelty seekers who just lose some level of turn-on after enough rounds with the same partner and must resort to fantasy to get up and over. The good news is that you've got company, masses of it. I wouldn't even call it a problem as much as a fact of life, and I wouldn't go assuming that your girlfriend never thinks about anybody but you or anything but what you're doing at that moment, unless she tells you so, and even then she could be fudging a bit to spare your feelings. You could ask or you could just keep doing what you're doing (it works, after all) and call it good.
Andrea Nemerson has spent the last 14 years as a sex educator and an instructor of sex educators. In her previous life she was a prop designer. And she just gave birth to twins, so she’s one bad mother of a sex adviser. Visit www.altsexcolumn.com to view her previous columns.