Don’t know why my body is failing us

I conceived my son naturally back in December 2018. I had him in September. Even though we were about to undergo fertility treatment God had other plans and gave us our miracle. I guess I had an unrealistic expectation that we would be able to get pregnant again easily since my body did it naturally the first time. We are on cycle 8 now and we go back to fertility doc once my next period starts. I feel so discouraged and upset. Don’t know why my body won’t get pregnant again. So frustrating.