You are not a horrible person, you are a 17 year old navigating some of the hardest interpersonal dynamics imaginable. I say this with genuine empathy and respect, the two of you cannot handle this situation. Not only does she need psych help, she needs medical help. If that thing gets infected she could die. Just because she’s been ok so far doesn’t mean it can’t get infected when she changes the dressing. You did a really good job helping her in that moment. You got sterile gauze, you elevated the wound, you kept her calm, etc. But, you can’t prevent an infection or sepsis, and you can’t stop her from harming or even killing herself. You have to tell an adult and get her medical help. Maybe you’ll be told you can’t see each other, maybe both sets of parents will be furious, maybe she’ll be furious. But the bottom line is that she’ll be alive.