Trying to conceive for almost 2 years

My husband and I have been trying to conceive for almost 2 years. We are under 30 and got pregnant only after trying for 2 weeks with our first. I don't know what is wrong and all the doctors say we are normal and said it will happen. It's been way to long and my heart breaks every month when my period starts. Out daughter is 5 and begs, cries and asks us daily why she doesn't have a sister yet. Every prayer she says she asks God for a baby sister but still doesn't understand why God hasn't brought a baby sister yet. My heart is broken and I don't know what to do. I am to the point where sex isn't fun and any talk of babies makes me wanna cry. I'm only 26 and shouldn't be dealing with this. This year has been even harder because so many coworkes, cousins and friends are pregnant or have had babies. I just want to have a baby!!!