Just remember that you have other options. Maybe, one night instead of having sex, just start doing "something" for him. This will usually lead to sex anyway. You could always start by giving or getting a backrub. You know that 90% of those lead to very enjoyable sex. Also, try having it in the morning before either of you rushes off to work. Sometimes just a different time of day makes a mood difference. Don't know if you are up for it, but watching porn together can put you in the mood when all else fails.