[Advice needed] The spark is gone.

So, some background, I am a 23 yr old female and my fiance is a 30 yr old male army veteran. (not sure if that's important but yeah..) He and I have been together for over three years and as far as sex went things were great we could barely keep our hands off each other and we would probably average anywhere from three to four times a week. There was fire and romance and passion, but as of recently like within the last year things have changed drastically. Now, we maybe do it once every three or four months he's become distant and distracted. However, when I try to talk to him about it to try and figure out what's wrong I don't get straight answers. I've asked him if it's me, and I get "no nothing's wrong with you it's me." I feel like I've tried everything so I'm turning to Reddit for help. How do I reignite the fire in my sex life?