Sex??

My boyfriend and I have been dating about a year and a half now and we waited 6 months until having sex he’s my first and hopefully my last but when we first started having sex I never said no to it and now it’s like we’ll go weeks without having sex because I’m never in the mood him on the other hand is always in the mood. I’m like never in the mood and when I am it’s for a split second and usually when we can’t do anything and it has led to several fight of where I promise him something then he gets home from work and I don’t wanna do anything anymore and it happens too many times. I love him so damn much and he deserves the whole world I just don’t know why I never want to have sex... help? Advice???