I'm not going to criticise your age so hear me out. At 16 your body hasn't fully developed to handle the stresses of carrying a baby. I sympathise with your situation, and can understand the reasons for wanting a baby, however I don't think you have analized the reasons for yourself. It seems like you want to make the family you didn't have.
Why don't you and your boyfriend get married now?
I'd hate for you to end up alone at 18 with one or more kids to support.
As for getting and staying pregnant, you have to go through all the stuff the rest of us did, try, try, try, and pray.