sex drive in the pits

Been marrried for 3+ years to the love of my life but i seem to be down on my sex drive. It's rare that I'm in a mood and at times the mood goes down the minute we get started. What do i do about this?! i still love my man and though its not butterflies in the tummy anymore.. we do have great sex when we do it! He's so understanding about it and never pushes me into it unless i want to and this makes me even more guilty. How do I get into the mood?!