Ana,
Thanks for sharing your story. First of all, let me say that I have never been in this situation before, but my wife has. She was 16 when she had her daughter and had to make a decision of what she thought was right. It is not easy, but remember that you are not only making this decision for yourself, but for you child. It is good that your mom has offered to help and cares enough about your and your child to want to help. I would say that you should stay with her. Even though we don't like to admit it all the time, our parents know things that we don't!

I know when my wife was in this situation, her mom was so helpful!
Although it may not seem like it, there will come somone who will fill the role of your child's father. They may not call them 'dad', but there are many great guys out there - don't give up. How despertely do you want your child to have a father? Do you want it enough to settle with an unstable guy? The most important thing is that your child has a stable home environment, even if it is with his/her mom and grandma.
You are in control. Don't settle just because you think that you have to have a father role and have a 'normal' life. Trust me, nothing about parent hood is normal. It has it's own wonderful suprises and you learn new things every day. It is a great experience...take time and enjoy it while you can. And never be afraid to ask for help. There are many girls who have been in this postition. Talk to them about it.
I hope all works out for you in the end.
chris_d