Im 19 and want to have a baby. I need advice!?

I'm currently 19 and my fiance is 21. We have been together for almost two years now and have been living together just as long. My fiance already has a 3 year old daughter. We have been talking about having a baby for a year now and have tried. I miscarried twice 5 months apart from each other both in my first trimester. It devestated us both because as soon as it was hitting us that we were having a baby it was being taken away from us for whatever reason in which god only knows. All I think about is what could have or would have and it tears me apart. I watch every baby show known to man and I get tunnel vision everytime I see a baby in stores. I've been so depressed about it for way to long and as I watch all these people who are only 15 and succesfully have healthy babies it just makes me envy them. I don't know where to go from here.. my fiance is willing to try again and so am I.. I just want to do it right this time. I also want to make sure im financially ready too. My fiance brings home 1300 a month and I bring home anywhere from 1400 to 2200 a month. Our rent is 600, and our bills come out to about 600. I spend about 40 a week in gas and we both smoke half a pack of cigarettes a day at 8$ a pack. We never go out, so we don't spend money anywhere else. advice.. someone please.