A lot of factors can attribute to that.
Have you tried conceiving on the best days according to your ovulation chart?
You also have to consider that maybe it's not just you who has the problem. The best thing to do is for BOTH you and your partner to consult with a GP and an OB/Gynecologist.
ps. Am sure a lot of people would say you're too young, and truthfully, where I come from, you are.
But it's different for different cultures. I think just as long as you're capable (emotionally, financially, etc), responsible and mature enough to handle it - I don't see why not. So many people wait too long and then when they reach their mid30s it's too late. I applaud you for making a choice, which is difficult for many of us to imagine. I just hope other people will respect that just as they have their opinions - you have yours too.
Sincerely... the best of luck :)