Stop thinking about it!
Remember that once you’ve started asking questions about your performance you could easily slide into a pattern of vicious circle where low self esteem triggers a failure, which, in turn, sends self esteem even lower and so on. There is no easy way out. Try to be a bit more positive about things. You were probably tired or stressed. Or you may need to learn some new positions and break the bedroom routine a bit. Surprise her with a new trick or a fantasy. And if you really have a problem, don’t despair. Every problem has a solution.