Where Do I Go Wrong

I have been trying to get pregnant for years now. It seems like everyone around me can have a child but me. The first time I was trying my brothers girlfriend had actually got pregnant along with my aunt then a few months later I get over my depression again and find out my cousin was pregnant that's not all so then my best friend and I start trying to have a child around the same time we bought fertility stuff and she got pregnant I didn't all this happened in a course of 3 years I had a thing where I would cry for a couple of days and try to avoid the person that was pregnant so I didn't have to pretend to be happy for them. I just wonder why I can't have kids. Where did I go wrong