Her previous (and only) therapist said she had OCD tendencies an focuses on the negatives constantly, to the point of them becoming debilitating. They said she may need medicine, but she's strongly against taking any and believes things can be fixed with the mind.
Oh boy, that puts you in a tough spot. It sounds like she needs treatment. Of course you can't tell her that because she'll think you're just making excuses for your own "bad" behavior (bad in her interpretation of things). You could approach someone else to talk to her (her mother or a friend) but unfortunately if they disclose that you asked them to talk to her that'll be interpreted as you making excuses AND gathering people against her. The best you can do for now is detach, give her time and space and hope that she comes to the realization that she needs med's to get through this.
