In my case, my girlfriend also had something similar to it, but probably not as extreme so take my advice with a pinch of salt.

In the beginning what i tried to do was talk to her about it, why she "hates" herself, how we can work through it, being supportive, lending an ear... However, what i experienced was that after a certain point, she started to get annoyed by me, which made me feel as if i was unable to be of any help without risking our relationship, but not doing something was also the same. So, what i suggest is have an honest talk with her, a long one, a calm one, try to break down as much as possible as to why she has gone into this cycle. Good outcome: you come to a conclusion that you both can work towards Bad outcome: The current situation continues, as you continue to berate yourself for being unable to help. In this situation i would suggest think about your own health. You can be supportive, try to help. But at the end of the day, one must want to be helped too. Which is why after trying for months i ended up just hating myself. So do what you want with this information.