Why do I not want sex as often?

Well, it could be two things. One, you have a naturally low sex drive & once a month is good for you. Two, you could have a medical condition that causes a lack of testosterone. Consult a physician. Do NOT use the over counter or herbal remedies for this issue without having consulted a doctor. They contain ingredients that can cause heart attacks in perfectly healthy individuals of any age. If she is saying once a month is not enough, see the doctor but also investigate ways to pleasure her that don't require an erection, yes they do exist & a lot of times doing them can induce the erection you are having trouble getting.