Afraid of having a baby..

So me and my fiance are trying to have a baby we have talked about and at first I was all for it but I still a little scared. Ive had my first child when I was 18 but my little girl past away due to some complication she had at 3 months old. Im really scared that this is going to happen again if I have another one. My fiance always tells me that everything's going to be okay but I still have these doubts. What can I do to help me get over those doubts?