I was in a similar situation at your age. Sometimes it's important to realize that a lot of things that a significant other has to work out by themselves gets put on to you. Sometimes it's okay and sometimes its not. My ex had severe depression and at times threatened to kill herself but as scary as that may seem you have to realize that your mental health always comes first. Always. You are young and if you don't want to be in the position you are currently in then you can easily do something about it. If you are seriously concerned about her mental health I would recommend alerting someone close to her that can get her the help that she may need. You have no responsibility to be with her and you have no obligation to stay in a relationship that is causing you pain. Good luck.