What do I do when.....

What do I do when I don't want to have sex but my fiancé does? He is always making moves towards it and gets irritated with me when I reject him. I don't want to cause friction (pun intended) but I really just have not been in the mood lately. And he doesn't listen to me when I say I'm not in the mood. I have been like this for well over a month. I don't even like for him to kiss me right now, I feel like he's cutting off my air supply when he kisses me sometimes. I just don't know what to do. Is something wrong with me?