Why is This Happening!!!

When me and my fiancé first met, we use to have sex all the time (when we could, we was on campus.) We've been together almost 2 years. We lived with my parents at first and he never wanted to have sex there because he wanted to "respect" my parents, which I totally understood so I let it slide. So instead we would just get hotels sometimes to have our privacy. We just moved into our own place and we've only had sex twice since we've been here (which is about 2,3 weeks now). I don't understand how come he never wants to have sex. He tells me he's tired from working, but I work too and I'm never tired. Yes we are in two totally different professions but I don't understand how he's always fucking tired and coming up with excuses on why he's not in the mood. Plus, when we finally have sex, it's quick and he's done for the night and just goes to sleep. I dont know what to do. Before I met him I was having sex all the time because I love having sex and the guys I was with knew exactly how to please me. I'm not a cheater and I would never cheat on him but I am so sexually frustrated!!! I don't like masturbating because I'm so use to sex it doesn't feel good to me. I don't believe he's cheating on me. I know he's not but I don't understand how he's never in the mood and when he is, he cums fast and it doesn't last long. Is there anything I can do to help his stamina or sex drive?