I think I'm infertile.

I've been married for roughly a year and a half now. I got married at 18, young I know. Anyway, fast forward to today... I'm 20, and we've been relentlessly trying to have a baby (he has a 5 year old, but I've been around since he was about 1). But I literally cannot seem to get pregnant. I'm starting to think something is wrong with me. Every month is just filled with disappointment and it's gotten to the point where he just wants to give up and stop trying.
I feel like I'm failing at being a woman. I'm failing him as a wife. I don't know what else to do. We've tried everything. Every test says nothing is wrong but there has to be.
Somebody please. Help. I'm desperate and just done. I'm tried of trying and being crushed every single month.