libido

I have only been having sex for like 4 months now and the guy I have been with is amazing, we are talking about getting married. At first I was down to have sex like all the time, but now I have kinda gotten less horny. He always initiates it and I ALWAYS tell him I'm not in the mood. What are some ways to kind of make me get in the mood? Not just foreplay, but like long term things?