I wouldn't keep taking her back. Obviously she does not know what the word commitment means. Do yourself a favor and distance yourself from her, she's bad news!
My girlfriend left me and dated a guy for two months then came back to me. I took her back and she broke up with me again and a week later is dating this guy. She's constantly telling me how happy this guy makes her?
Move on. This one is not a keeper. Find someone who knows what she wants as this girl seems to just be playing around.
You need to move on with life, why put your self through all her torture?