She doesn't want you back because she's done with you but at the same time she'd hate to see you happy/with someone else because she considers you 'hers'.
Fuck her. Call her out on every little thing she did wrong, every little thing that annoyed you. Send her a massive text about how much of a cunt she is, tell her you're already seeing someone much hotter than her, does all the things she didn't and things she did better. Then block her number, block her on Facebook and let her die a little inside.