I don't know how to tell my best friend that I don't want to room with her in university?

I'm an extreme planner and i've decided to get an apartment and live there from university on. I'll have one roommate. I know who I want as my roommate and it's not my best friend. I love her like a sister but her life choices suck. She's failing in high school, doesn't have a job, and has no ideals for her future. I don't think she's even planning to go to university.  I'm worried that she won't be able to pay for her half of rent and I don't have the money to help her out. She acts like she's assuming that we'll move in together, but I don't know how to tell her that I don't want that.