Why can't I break up with my girlfriend?

She often treats me bad, is emotionally distant and unavailable, is never there when I need her but expects my life to stop when she has any problem what-so-ever, I’m relatively sure she’s at least toying around with one or two other people, I don’t see any future for us and the sex isn’t even that good!! So why can’t I cut myself free of her? Why can’t I just end it and walk away, or just let her walk away? What the hell is my problem??

(We’ve been together almost a year).