Your brain is your most erogenous zone--it is your brain that stimulates you to feel sexual. At the same time, it is your brain that stimulates you to feel tired or stressed. With this in mind, it makes sense that there are times when our bodies will not respond because our brains are too engrossed with worry, stress, work, or whatever is keeping us down.
I suggest that you take a serious inventory of yourself to discover exactly when you are not feeling sexual. If it is stress, ask yourself what you can do about the stress. If it is the time of the month, try to discover if you can do something to override the physical reactions. If it is a situation, a word, a glance, ask yourself if you can resolve the situation.
It is best if you can discuss these issues with your boyfriend in a neutral place and at a neutral time. Do not bring this up in the bedroom or when he is making advances towards you. You could explain that the stressors in your life can sometimes drain your energy. Explain that it has nothing to do with him, but you feel so burned out that sex can be too much for you. Ask him to understand and to go with YOUR flow. Tell him you are fighting these moments and hope they will become less frequent. Tell him you love him.
It's also essential that you remember that sexual pleasure has a built-in health benefit. It is physiologically important to relieve your body of tension, and the orgasm is one of the most practical and powerful ways to do so. At the moment of orgasm, all muscle tension releases, blood flows with ease, there is freedom from thought and a moment of joy--and it is amazingly healthy. At that moment, the body is free of negativity, acidity, and stress. Take advantage of the benefits of orgasm rather than depriving yourself of this miraculous and essential release.
To conclude, use all of your emotional tools to be loving and caring to your boyfriend; AND be sure to be especially loving and caring to yourself. It is your brain which will help you through this dilemma, and it is your brain which will take you from the stress and bring you to all of the pleasure you deserve.
Finally, I appreciate Dr Altman's explanation of the physiological importance of orgasm; I may cut and paste it into an email and send to my wife. (Kidding)