The first several times we had sex, it was awesome/amazing. Later on, when I became frustrated with how she was acting and how selfish she was/is in bed, I became kind of jaded with having sex with her. She's a fan of the starfish position, and demands that I lay on top of her while I fuck her. It's not particularly easy or comfortable, and definitely not my favorite position.
As these feelings of negativity grew, I became less physically attracted to her and sex became more of a chore, part because I'm doing all the work, part because I'm not physically attracted to her as much anymore.