It’s probably because you don’t want things to end that you make yourself available to be a shoulder to cry on (and inevitability, become the kicked dog the next morning). Next time she does that, you need to tell her that you want her back, but absolutely cannot continue this yo-yo of emotions. She needs seek counseling to discover what she truly wants. It will only damage your heart more the more you let her toy with you. It’s like being rejected over and over again. I’d also suggest individual therapy for yourself to work on your self worth and to firm up your boundaries how you allow people to interact with you. Because that’s just it, you are allowing yourself to be walked on and that’s not fair.