A. No, there's absolutely nothing wrong with you. The traditional male-centred view of sex is straight forward: desire is followed by sexual arousal, then sexual excitement, then climax. If only it were that simple for women! Recent studies suggest that some sexual response cycles, especially women's, seem to be more complex and circuitous than we originally thought. Rather than sexual desire, the cycle may start with an emotional desire (to be loved, for example, or found attractive). Then distracting thoughts, like a work deadline, annoyance over a partner failing to do the laundry or feelings of unattractiveness can inhibit arousal and climax.
So rather than telling yourself to 'get in the mood', it might be better to just do it and focus on the physical sensations or activities that make you feel sexy. It goes without saying that you should never do anything you don't want to. But most of the time it's not that we don't want to, it's that we're too tired, stressed or annoyed - so just doing it can get you over that hump, as it were. (It's like working out - the hardest part is getting yourself to the gym, but once you're there, you're glad you went.) Plus, the more sex you have, the more you want, because sex increases your testosterone levels, which actually increases your desire.
Take it from Yesgirl - we should all say yes to more things, but never be forced or coerced into doing something you feel uncomfortable about.