The good news is that it can be done, and it can actually be done fairly easily. And, guess what? You have the power to do it. It just takes a tiny little effort to say, "yes," instead of "not tonight." "Yes," even if you are still angry, even if it feels weird, even if you are really not in the mood and don’t have the energy, learn to say yes to your relationship. It's worth it, isn't it?
I know what you are thinking. Easier said than done, right? Well, there is a way to get your needs met too, in the process. Communication has a lot to do with it. Yes does not necessarily mean that the man must achieve a full erection, or the woman achieve a full arousal. Yes does it even mean that intercourse has to occur, or orgasm has to be achieved. Yes may mean kissing each other for 5 minutes, or taking off your clothes and lying together naked and just talking about your day. Yes could mean taking a bath together, or just lighting some candles and breathing together. Yes could mean reading erotic novels together or watching an erotic film, going to an adult store, or doing a sensual massage together. Yes could mean talking about a fantasy or creating one together. Yes can mean a non-sexual role-play that is still arousing. Yes could mean mutual masturbation.
My point is that Yes does not have to be sexual but that it should be sensual and arousing, exciting and fun, romantic and titillating, and it should be comfortable. Sometimes the thought of having to perform sexually with someone who has become our best friend, can seem down right daunting. You have permission to take it slow, and to take it where you want and need it to go. Your partner has permission to join along on the ride, with no expectations. No one has to have sex, no one needs to orgasm, just enjoy each other's company in a sensual setting. I do however strongly recommend getting naked, and women this can mean slipping into something sexy first. I also recommend tuning off the TV, computers and cell-phones. Get creative, break the cycle not just of the comfort, but of your ideas of sex, sexuality and sensuality which contribute to the comfort default, the perceived comfort. Learn to connect in other intimate ways. It takes 5 minutes a day.
If you still feel like you are having troubles, there may be other roadblocks. Talking to a psychotherapist or sex therapist can always help.