sex life is nonexistent

My boyfriend and I have been together for a year and a half and it has been the most AMAZING time of my life. We have been living together now for about 4 months and I absolutely love it. I love him so much, there is no question that I love him. He's my best friend and the most amazing guy I've ever known. I know that I want to spend the rest of my life with him. The only issue is, we never have sex anymore. He's never in the mood. All the things that used to drive him crazy for me (like me walking around the house naked for example) don't do anything for him anymore. Almost every time we try to have sex he either can't get hard, can't stay hard, or can't finish. I don't know what to do. I'm an extremely sexual human being and I've never had this happen to me before. We are extremely adventurous in bed so I know that it's not too vanilla for him or anything. And we are both so young! I'm 20 and he's 21. We shouldn't be having these issues. Ugh I'm just so frustrated and I don't know what to do. I haven't had a good fuck in months now and it's killing me. Does anyone have any advice??