Do you watch porn? If yes, that's the problem. It re-wires your brain to require visual stimulation equal to that of a harem, a luxury only kings were privileged to enjoy in the past.
In our age, every man can live like a king in his mind, except it's not real, and when the king leaves his virtual harem to have a relation with a real life flesh and bone woman, she can not give him the visual stimulation that he is used to.
The only solution is to stop watching porn. Your problem will go away after enough time away from porn.