Age is nothing but a number???

My fiancé is 15 years older than I am and I am 39.... sex used to be great when we first started dating and before we moved in with each other but not it's almost nonexistent.... what should I do???? I mean I like sex and when we finally have it, it's great but the dry spells are becoming a bit much.  Once or twice a month isn't getting it for me.