Boyfriends sex drive sucks

My boyfriend and I have been together for just about 2 years now and our sex life is terrible.  I couldn't tell you how many times he has turned me down for any kind of sexual advance. We get along extremely well and we do live together but I really don't understand why he is just so mute to having sex.  Here I am in my prime and ready to pounce but it is always an excuse. Sounds like it should be the other way around. It's extremely tiring and I need help on what to do to change it. I'm frustrated!