Failing sex life in relationship

My boyfriend and I have been together since June of 2015. Around November, I realize that he didn't want to be intimate with me anymore. I tried initiaiting it, buying lingerie, and nothing worked. Around January I talked to him about it and he made it seem like I was crazy, and that he always wanted to have sex with me, but we just never had the chance or he was busy, which is not true. We have had many chances. I waited some more and talked to him again in March. He gave me the same answers again. He gave me more excuses. We had sex about once a month, and it was  when I initiated it only. There were a couple of times that he sighed and said stuff like, "Are you kidding me? Right now? It's so late", etc.  it's now almost November and the last time we had sex or did anything sexual was MAY. I am sick and tired or waiting and I've had so many conversations with him and I feel like I have done everything I could do. I even spent so much money on lingerie that did nothing! He said he was sweaty and dirty from working and pushed me off of him. He eventually felt bad and said it was a mistake but it still makes me sad every time I think about it. He definitely isn't cheating because I'm always with him from Friday- Monday and I know who he works with and stuff so I'm just stick at this point I the relationship. What should I do? He just makes excuses and I feel stupid for always trying to bring it up or ask him.