Maybe it's me

My fiance ' exes had no issues with getting pregnant( he has kids with both his exes). They got pregnant quickly into their relationships and we have beeb trying for 8 months and nothing.... Sometimes I think maybe he is the reason we aren't getting pregnant but I think it's my fault. I'm overweight and I have terrible anxiety. I just want to give him a child that he can enjoy ( his exes are not nice ladies- I've witnessed it, it's not just because he told me so).