Oh my gosh, teen love. It's so hard! My son is 17 and liked/sort of dated a girl a year younger. He really liked her. He's insecure. They weren't ever at the point where he'd accuse her of cheating as they were more casual than that but he cared for her deeply. She has some mental health issues and sometimes cuts. Scary. And my son is also prone to depression. She ended the relationship and he is so hurt. He's also had some very scary thoughts like your daughter regarding his life. I'm on constant alert. I've just been trying to show him constant compassion and keeping an eye on him. He misses the girl terribly. Break ups always hurt. But teenagers. Whew, they aren't always equipped to handle it well even in the best of circumstances. Add on mental health challenges? It's hard. Does she still see a therapist? I'd gently help her distract herself. It takes time to move on but hopefully she does soon.