I feel terrible

I feel happy for my friends who've just popped or who've just conceived. But there's always a but... Sometimes it's overwhelming, like everyone is getting pregnant but me. Why is it so hard? Is there something wrong with me? 
​People tell me I'm too impatient and I shouldn't be too worried. I'm trying my level best to be positive but it's not easy, when everyone around you seems to get pregnant on their third try...