Other people have talked about their relationships, and those can go bad at any time, from a week in to 30 years or more in. A relationship is a lot like a garden: you have to keep pulling the weeds, and watering/feeding if you want it to grow. And that care is required as long as the relationship exists.
But your partner is only half of the equation, at most. You also have to take care of YOU. At any time of life, but especially once you hit your 40s, staying active and healthy is VITAL to having a good sex life. I learned this a couple of years ago after a sports injury kept me from exercising for a few months. I found my sex drive plummeted and only came back after I started exercising regularly again. I've also found my libido drops off if I overeat, or eat unhealthy foods. The "garden of the self" has to be tended, too.