I won't judge your situation, cause I don't know the specifics. You seem to be pretty mature for a 15 year old, and if it's been good livin with grandma, I'd try to stay there if I were you. Take as much time as you can living there.

You need to get the skills to be an independent adult, and that means some education (GED, trade school, college) and job skills. Lots of kids with tough childhoods end up having good adult lives. Think it through and make the best decision for you.