Divorced, engaged and ttc

I left my ex husband 2 and a half years ago. I've been with my fiancé for almost 2 years. My ex has now gotten 2 women pregnant without trying within a year of each other. Fiancé and I are actively trying to get pregnant and it isn't happening. What am I doing so wrong? Why is it that my ex can get 2 women pregnant that don't want more kids? I have a million issues due to my ex. Now I have even more. Life just isn't fair.