Several things come to mind here. Remember that at this time in your life you are going to be feeling kinda mom-ish because your hormones are going crazy and it's your natural instincts awakening. That combined with your loss is going to make you feel very emotional about babies and the prospect of having one yourself. You are in a very vulnerable position and having a baby right now ould be an emotional and impulsive choice but not the best considered choice.
You are only 15 and you don't yet have the life experiences needed to bring up a child by choice. Many girls do end up having children in their teens, yes, but few of them actually chose to go out and get pregnant - for many it was an accident. If you are going to consciously choose to have a child then you should make sure that you have several things first. Remember this is another human being we a are talking about - not a pet. You have to remember that the responsibility for having a baby is on you. It is not a child's responsibility to love you - it is your responsibility to love a child. I feel distinctly like you are thinking of this baby entirely from your own perspective and what you would want from it and not vice versa, which is how it should be. It is simply unfair to bring another human life into creation for your own satisfaction.
Do you have the financial stability to bring up a child for 18 years? Do you even have any credit history yet? Can you even get credit yet? Do you have a job to bring in money? Raising a child and providing for it for so many years requires an enormous financial commitment which you aren't ready to make yet at all. Your parents and family would end up having to support you and the child would suffer as a result.
Where will you find a father for this child? How many boys your age want to be fathers? And you can't simply have unprotected sex and not tell them - that would be completely unfair on them if ever they found out that you had had a child. it would burden them with a responsibility that they might have never intended to undertake just because you wanted to. And again - that's not fair o the child. The child at least deserves a chance of having a loving father around if it's at all possible.
And what about your life? You are only 15. There is plenty of time yet for you to have a life of your own as well as have children. You need to have that life as well so that it provides you with the experience and wisdom that you will need to raise a child and teach it the necessary lessons of life. If you are still a child yourself and have missed out on valuable experiences then how will you be able to teach another person those things? You could see your life for the next ten years or whatever as being training for child-rearing.
Your comment about thinking you will die young is rather ironic don't you think? I mean I understand that you may ave this feeling, but for a start it is completely unfounded and no one knows if they will die young or old. However if you are going to die young how is that a reason to have a child? SO if you're going to die young then you want to bring a child into the world just for your own benefit and then what? Die on it? Leave it without a mother, and a father who never even knew it existed? If you really believe you are going ot die young then the last thing you should be doing is bringing a dependent into the world.
And remember that is what babies are - completely and totally dependent on you. You need to be able to provide an environment in which it can grow and flourish - in which you can nurture it. You simply can't do that yet.
Try and seek some professional help for your loss. Maybe a lot of these feelings do stem back to your miscarriage and your loss there and some professional help in the form of therapy might help you to find more healthy ways to move on with those. Perhaps if you do that then you will be able to bring a child into the worl done day in the future when you are fully set up and ready.