I'm infertile and still haven't told my fiance
Is there a reason why you haven't told him?
I'm too scared that he'll break down.
It's reasonable to be nervous about sharing your infertility with your fiance, but keeping it to yourself is inappropriate. You should tell him because this is such an important topic and he deserves to know about it.
I'll tell him later.
It's okay to wait a little bit until the moment is right, but the longer you wait, the harder it will get.  Your fiance deserves to know about your future together before you get married.