I suggest you tell your fiance about how you feel and see how he responds. Before you do though, take some time to consider 1) whether you would be WILLING to work on the sex drive issue - if it's something you think you might desire but there's just a barrier that impedes you - or if you strongly believe sex is something you don't want in your life and you're sure about this (I am not judging in any way and I am not saying one option is better than another in any way - it's about how you feel), and 2) whether there may be a chance you might want children in the future; you're 31 years old now but women sometimes have children only in their late thirties, early forties and beyond (you can also consider adoption as an option, etc.). As I said before, make up your mind about how you feel about these two issues and then talk to your fiance. He will share his view and hopefully you two can find a way to compromise. If no compromise can be made, maybe breaking up is an option that will make both of you happier in the long run.