I feel like I shouldn't even be on this earth and I'm only 15!!?

I'm sorry you're having it so rough right now. I do want you to know things will get better. I can promise you that. Maybe you could talk to your grandparents about it. Be sure to tell them that you love your parents and your new brother. Another idea is to ask your parents to sit down with you and have a discussion. Ask them to pick a time that's good for them, when they"ll have time to listen to you. Sometimes there are no easy answers, but remember you will be there only a few years longer. Please keep up your grades and make plans for the adult part of your life.